mixing oil viscosities

[CStamm]

Quick question guys, is it ok to mix oil viscosities? I have a 98 with oil pressure hovering at or below 10 psi at hot idle. I have been running Mobil 1 full syn 10W-30. I was considering putting in 4 quarts of this and the rest with Castrol 20W-50 dino, to see if it helps the oil pressure. Any concerns with doing this? Also, I know it's a band aid and I could probably stand to do the rod bearings.

 

[titan4u2c]

Dunno if I understand you correctly about viscosities, as in syn vs reg oil? That's a no-no you can't go back to reg if you switch over to synthetic. But as far as the weight you can mix like a 10w30 and a 40 or what have you. Wow, 10psi is low. I have always ran 6 quarts of Castrol GTX 20-50 in my '91 and have almost 300k with no issues. So maybe you have a seal or other issue? What kinda filter are you using? I'm a firm believer in marvel mystery oil and now and again I add that restore additive and no pressure probs. Maybe the sending unit? Sorry this isn't much help but hope you get it resolved.

 

[87manche]

Dunno if I understand you correctly about viscosities, as in syn vs reg oil? That's a no-no you can't go back to reg if you switch over to synthetic. But as far as the weight you can mix like a 10w30 and a 40 or what have you. Wow, 10psi is low. I have always ran 6 quarts of Castrol GTX 20-50 in my '91 and have almost 300k with no issues. So maybe you have a seal or other issue? What kinda filter are you using? I'm a firm believer in marvel mystery oil and now and again I add that restore additive and no pressure probs. Maybe the sending unit? Sorry this isn't much help but hope you get it resolved.

SPOBI
You can go back to conventional oil after you go synthetic. It's all got an SAE number and it all has to mix per those guidelines. If you're going to do it though I'd just go full 10W40 or 20W50, not much point in mixing it.
read this:
http://www.advanceautoparts.com/english/youcan/html/ccr/ccr20050201so.html

 

[5-90]

You can mix viscosities without trouble - I've done it for years. For instance, if I have a leak that I can't fix right away, and I'm due for an oil change, I'll replace a quart or two of 10W-40 with straight 50 weight or something around there, and that brings up the base viscosity of the oil.

You can mix synthetic and regular oil - I've done that as well. Hell, you can probably still BUY it that way - Valvoline and Castrol used to do a "half-and-half" oil.

And, (I haven't done this yet...) I see no reason why you wouldn't be able to switch back and forth between synthetic oil and dead dinosaur fluid. As has been mentioned, they have to meet SAE specs either way - it's just that the synthetic base oil isn't two million years old. It's probably not even two million minutes old. You might note a few leaks when switching from the one to the other - because synthetic is somewhat slipperier than dead dinosaurs, and can get past seals - but it carries seal conditioners, and it may stop shortly. Also, because of the extra seal conditioners, you may note leaks after switching from synthetic to dead dinosaurs - which may not go away, but a gasket change should see to that. However, it won't cause any trouble with internals or hard parts - like bearings, shafts, lifters, and such.

Lubricating oils pretty much share base oils, within the petroleum family. The main difference is whether it's manmade or dead things, and where in the fractioning tower they pull off the base oil. Gear oil, for instance, is essentially the same base as transmission fluid - it's just that the 10 viscosity base for ATF comes from somewhat higher on the fractioning tower than the 80 viscosity gear oil base...

Oil is "cracked" into pretty much everything from asphalt to LP Gas - with all sorts of things in between.

5-90

 

[XJ88User]

Yes you can still buy half and half oil, saw some when I was changing my diff oil about 2 weeks ago.
The engine a XJ doesn't see the compression and heat loads that would cause problems to a half an half oil. Things like jet turbines have to worry about the compression, wear and fractioning of oil that has mixed viscosities, that why they monitor oil degradation and deposits when fixing them.
You could do the same to your engine but changing the oil at a defined interval like 3000 miles with a good quality oil will take care of most problems for us in the general population.

 

[RNMCGUIRE]

Wow, some of the replies on this site are just horrible. People should make sure they know what their talking about before they post information. As far as viscosity, I was instructed in school that it's not ok to mix different viscosity oil because it can cause the oil to react negatively. Different brands are ok but not different weights. As far as synthetic goes, they are fully compatible with regular oils and you can revert back to standard oil after using synthetic.

 

[CStamm]

hmm conflicting responses here. I am not afraid of going from dino to synthetic and even back to dino if i so choose. I belivev that to be a myth from much reading on the subject. I am going to replace the sending unit soon, but I'm not too surprised at the low pressure. It only has 118K miles, but I know the previous owner beat it (my neighbor). Then again it only cost me $2700 and I drive it 500 - 1000 miles a week for my job. I wanted to switch to Mobil full synthetic 10w-40, but it isn't made. RNMCGUIRE is kinda scaring me now with his comments on mixing viscosities.

 

[Dr. Dyno]

Quick question guys, is it ok to mix oil viscosities? I have a 98 with oil pressure hovering at or below 10 psi at hot idle. I have been running Mobil 1 full syn 10W-30. I was considering putting in 4 quarts of this and the rest with Castrol 20W-50 dino, to see if it helps the oil pressure. Any concerns with doing this? Also, I know it's a band aid and I could probably stand to do the rod bearings.

Mixing viscosities isn't really going to help since the oil pressure's already very low. Since it seems that you've got excessive bearing clearances and you'll need to replace rod/main bearings sooner rather than later, don't bother using synth anymore and just switch completely to Castrol 20W-50 dino oil. This oil might just be thick enough to raise the oil pressure a tad above 10psi and buy you more time before you replace worn bearings (plus oil pump and rear main seal while you're there). You can go back to using Mobil 1 after you've treated the disease and got the oil pressure back up to normal.

 

[5-90]

Wow, some of the replies on this site are just horrible. People should make sure they know what their talking about before they post information. As far as viscosity, I was instructed in school that it's not ok to mix different viscosity oil because it can cause the oil to react negatively. Different brands are ok but not different weights. As far as synthetic goes, they are fully compatible with regular oils and you can revert back to standard oil after using synthetic.

There is no real reason whatever to NOT mix oil viscosities - the base oil is chemically the same, the difference is the viscosity improver package that they put into it.

As I recall, the viscosity improver additives are a mixture of assorted long-chain liquid polymers - the different proportions are for different "weight ranges" or viscosity ranges. I seem to recall that the base oil has a viscosity of somewhere around 3-5, which is then increased by adding the polymer package in varying proportions. Heavier "weight" oils may have heavier bases, but they are otherwise chemically similar (the bases for automatic transmisson fluid and engine oil are functionally the same, and gear oil is pretty much the same, but comes from a heavier fraction.

I've torn engines down - partially or totally - after a life of mixed viscosity oils, and I've not seen any ill effects.

Granted, it's not recommended by the manufacturers - but they don't like people who think they know more about engineering, petrochemistry, or metallurgy getting that involved in vehicle operation (they usually don't, but that's another story. Also, it gives them a legal "out" if you haven't followed the manual precisely.)

In a working engine with all the seals in good shape, there's no real reason to start mixing oil viscosities. However, to temporarily "solve" a problem that you're going to fix in the not-too-distant future, it's perfectly alright.

Might I enquire about which school you were taught not to? I'm honestly curious - my experience has shown otherwise, and I'm wondering.

5-90

 

[drifto77]

Glad to see other people run 20-50 besides me.
On another fourm people say its too heavy for the 4.0.
I use it in everything.

 

[titan4u2c]

Hmmm people that go by the science in an article by advanced vs. what their reputable mechanic should tell them. Guess it all boils down to what you wanna do with your ride, seems like this is another half a dozen this and so on that. Difference of opinion maybe but I don't have to drive or work on anyone else's ride either so point taken here. But it even says in that article that the companies themselves don't recommend it, hmmm that really stood out. So I guess you can do it yeah, anything is feasible, question is will it be worth it? And yeah if you change out now the newer syns are better and will mix or go back whatever. But I was refering to earlier in the life of the vehicle as they stated with the use of syn and going back. But yeah I got no problem running the dino 20w50 in the xj and like I said 300k, how many people r there yet? lol And syn in my Titan and I'll pull the heads and compare with anyone that went from dino to syn and back. Would be interesting to see if any of the manufacture's claims were worth a crap. haha Good input fellas.

 

[ChicksDigWagons]

Hmmm people that go by the science in an article by advanced vs. what their reputable mechanic should tell them. Guess it all boils down to what you wanna do with your ride, seems like this is another half a dozen this and so on that. Difference of opinion maybe but I don't have to drive or work on anyone else's ride either so point taken here. But it even says in that article that the companies themselves don't recommend it, hmmm that really stood out. So I guess you can do it yeah, anything is feasible, question is will it be worth it? And yeah if you change out now the newer syns are better and will mix or go back whatever. But I was refering to earlier in the life of the vehicle as they stated with the use of syn and going back. But yeah I got no problem running the dino 20w50 in the xj and like I said 300k, how many people r there yet? lol And syn in my Titan and I'll pull the heads and compare with anyone that went from dino to syn and back. Would be interesting to see if any of the manufacture's claims were worth a crap. haha Good input fellas.

I don't know about you but anything the oil companies say I take with a grain of salt. Why would the oil companies want you to stick to synthetic and not go back to dino oil? Its called Profit.

For what its worth, I've mixed just about anything and everything at one point in time or another and never had a problem.

 

[87manche]

Hmmm people that go by the science in an article by advanced vs. what their reputable mechanic should tell them. Guess it all boils down to what you wanna do with your ride, seems like this is another half a dozen this and so on that. Difference of opinion maybe but I don't have to drive or work on anyone else's ride either so point taken here. But it even says in that article that the companies themselves don't recommend it, hmmm that really stood out. So I guess you can do it yeah, anything is feasible, question is will it be worth it? And yeah if you change out now the newer syns are better and will mix or go back whatever. But I was refering to earlier in the life of the vehicle as they stated with the use of syn and going back. But yeah I got no problem running the dino 20w50 in the xj and like I said 300k, how many people r there yet? lol And syn in my Titan and I'll pull the heads and compare with anyone that went from dino to syn and back. Would be interesting to see if any of the manufacture's claims were worth a crap. haha Good input fellas.

Since that first line was directed to me I'll answer a few things.
All oils sold have to meet the SAE standards, if it's an SAE oil then it's an SAE oil, it HAS to mix with other similarly SAE certified oils. The base stock does not matter, only what the SAE standard says it has to do.
The key is that they both are meant for automotive use. You can't go mixing any oils together, they have to have the same API ratings. The current rating for API automotive is SL, so you can mix any two SL rated oils.
So I don't see a problem in mixing oils, some people much more intelligent than me have already done all the engineering and stuck the results on the top of the bottle. Is it soemthing I do everyday, no, but if I have an odd quart about and need to top the oil off I'll do it.
My 4.0 has 250K miles on it

 

[XJ88User]

As far as viscosity, I was instructed in school that it's not ok to mix different viscosity oil because it can cause the oil to react negatively.

I have seen synthetic used as a straight weight, mostly because it covers such a wide temperature range, but I have to wonder if this hold true across the spectrum of oil uses. So your saying there is no need to sell any multiple viscosity oil any where for any purpose and you have proof of this?
I'm sure things have changed since I have sat down and did any in depth reading about this at the degree some one in school has been, I too would like to update myself as it seem my information is out of date. If you could post an article or a link I'd appreciate it.

Oh just for a side note the XJ just crossed 340K this week for me.

 

[Matthew Currie]

Like 5-90 and 87Manche I've been mixing viscosities for a very long time, though I don't do it when I can avoid it, simply because I prefer to find a viscosity I like and then buy a case. Obviously if you mix viscosities you will not know exactly what viscosity you've ended up with, but I doubt there will be any other harm, and I certainly have never had problems with numerous very high mileage vehicles.

However, in response to the original post, I think that it's a poor solution to the problem, in part because as I said you won't really know what viscosity you're ending up with, so it will be hard to evaluate the results. If it's due for an oil change anyway, I would put in the best filter you can afford (In my experience you may get a couple more pounds from a Wix than from a generic one) and fill it with all one thing, such as 15w-40 dino oil. You'll have an answer to your question and no harm to the engine even if you decide to go back to synthetic next time.

 

[Eagle]

Wow, some of the replies on this site are just horrible. People should make sure they know what their talking about before they post information. As far as viscosity, I was instructed in school that it's not ok to mix different viscosity oil because it can cause the oil to react negatively. Different brands are ok but not different weights. As far as synthetic goes, they are fully compatible with regular oils and you can revert back to standard oil after using synthetic.

You have it reversed. It is okay to mix viscosities, but you should not mix brands because the chemical additive "packs" are different and may not be fully compatible. However, adding one or two quarts of 20W50 to 4 quarts of 10W30 will not do much to improve oil pressure. It would be better to just do an oil change and switch over the something like Mobil-1 15W50.

And be sure not to use a Fram filter.

 

[lawsoncl]

I'm surprised no one has mentioned that the orignal poster should verify that he does indeed have low oil pressure. The oil pressure guage isn't very accurate to begin with and it's not unusual to have a bad oil pressure sender/sensor.

 

[UNCC_99XJ]

I've switched back and forth a couple of times in my XJ and have no problems with it. Went from dino to synth., back to dino again, and now im back to synthetic. Reason for the switch back in the first place was I developed a rear main and a oil pan gasket leak at the same time and I wanted to make sure it was the synthetic that was making it leak, which it is. However, I noticed a huge difference in how better the engine ran with synthetic in it, and I do pretty much all around town driving (commute to school is about 3 miles, and commute to work is about 2 miles).

 

[Rick Anderson]

....the viscosity improver additives are a mixture of assorted long-chain liquid polymers - the different proportions are for different "weight ranges" or viscosity ranges. I seem to recall that the base oil has a viscosity of somewhere around 3-5, which is then increased by adding the polymer package in varying proportions....

Those long-chain Polymers are coiled, when cold they are tightly coiled and smaller, as the oil gets warmer the long-chain Polymers start to uncoil and get larger, the increase in size resists flow and increases the relative viscousity.

IIRC, these long-chain polymers are the most likely to burn of all of the additives, they'll coke up and leave deposits behind.

The closer in low/high viscousity an oil is, the less it needs those long-chain polymers, since there is less of a change in viscousity, thus more of the base stock oil is used for lubricating and less additives to burn, coke and deposit on the engine.

I've always seen that advice, to use the narrowest viscousity range that will cover the operating temps for the vehicle, will always serve you best.

IIRC, Synthetics don't use the Long-Chain Polymers (LCP) to achieve the multi-viscousity. Not sure if they engineer the base stock to have neccessary viscousity range as a natural property or if they use some other additive, but this is one of the biggest reasons Synthetic is so much cleaner. Less additives, especially none of the additives that burn easily, that are in conventional oil.

Might I enquire about which school you were taught not to? I'm honestly curious - my experience has shown otherwise, and I'm wondering.

I've mixed different viscousity oils as well, but not as a regular practice. 10W30 and 15W50 Mobil1 Synthetic, it gave me the very desirable result of only slightly higher cold oil pressure with higher hot oil pressure. (Straight 15W50 was giving me way way too high cold oil pressure).

BUT, I stopped because I had another person tell me mixing different viscousity oils was a BIG BIG NO NO, and that advice had come from industry insiders. BUT, he couldn't supply the reasoning why not, again we hear here its a NO NO, but no can tell you why or what goes wrong when you do it. I'd like to know also?

 

[Dr. Dyno]

I'm surprised no one has mentioned that the orignal poster should verify that he does indeed have low oil pressure. The oil pressure guage isn't very accurate to begin with and it's not unusual to have a bad oil pressure sender/sensor.

Yes, that's possible so as a first measure I would suggest replacing the oil pressure gauge sending unit. My gut feeling is that it isn't going to make a difference but at least CStamm will know that the oil pressure is indeed low.
The next thing would be to change the oil filter (to a Mopar or Wix) and use a thicker oil (20W-50 dino). It might raise the oil pressure up to 15psi at hot idle and keep the engine going for a while longer until he does the permanent fixes that I suggested.
On the general topic, I don't see any problem mixing oil viscosities as long as they're the same brand e.g. Castrol GTX 10W-30 and Castrol GTX 20W-50, Mobil 1 10W-30 and Mobil 1 15W-50.