mixing oil viscosities

[zj to xj]

:explosion Mixing viscosities is not the best idea. Chemical engineers have carefully engineered these oils with a unique base along with friction modifiers additive packages and detergents. Each Brand is uniquely different as well as each different viscosity. There are enough different grades of oil out there to not need a homebrew fix. As far as synthetic to dyno switching it is okay. But the best thing you can do for your motor is pick a brand of oil and particular viscocity and syth or dino and stick with it....... just my .02 :read:

 

[Thayer]

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.

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.

Aren't these statements contradictory? Hmm. I think the mythbusters need to handle this.

 

[CStamm]

I will change out the sending unit this week, maybe try and get a mechanical gauge on it too. At hot idle it seems to fluctuate between 10 and 15 psi for no reason, only if it is idling a minute or two. As in, constant RPM's, 30 seconds gauge reads about 10, then it will move up a few psi, then drops again. Always 40 psi or higher over 2K rpm's on the highway. I can't really complain, I put in up to 6 hours of seat time a day and it keeps chugging away.
Thanks for all of the opinions on mixing viscosities though.

 

[5-90]

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.



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?

I don't mix viscosities as a regular practise either - only when I need to "band-aid" something until I can fix it right (which usually happens within a fortnight - month at the longest.) It's still something I'm not entirely comfortable doing - but I'd be a damn sight more uncomfortable about letting my oil sump run dry if I don't!

However, I prefer to just fix leaks, rather than "afro-engineer" a solution and stick with that over time. Fixing them is usually far cheaper anyhow.

If someone tells me that I should or should not do something, my next question will invariably be "Why?" If I don't get a satisfactory answer (which, in my world usually means a logical derivation from readily-verifiable premises and grounded in real-world physics and chemistry...) I'll just go on my merry way. I'm always willing to be convinced, but it does take some effort to convince me - "Because" has never worked. You do have to supply something after the "because" with me - and that used to drive my old man nuts. (No, "because I'm bigger/older/meaner/whatever never did work worth a damn either. That's either self-evident or inaccurate, and has no bearing in the real world anyhow.)

Thayer - on the surface those statements may seem contradictory, but not as much as you might think. SAE standards have to be met in order to get an SAE rating, and API standards have to be met to get an API rating, but neither sanctioning body says HOW those standards have to be met. So, like on tests in school, there can be several different ways to get a "correct" answer - and that's the rub. Each company probably has a different package with different contents for additives, and some of those additive components may be mutually antagonistic.

SAE/API specifications govern viscosity, viscosity ranges, anti-wear -sludge -acid -whatever additives - but that doesn't mean they all have to be done the VERY SAME WAY. You may use spit to dampen your whetstone, I use honing oil. Both are acceptable, but not interchangeable. I have "field stones" that I will use some water-based whetting on (whether spit, water from a canteen, or just dunking them in a handy creek) - but they are separated from my shop "oil" stones - which will never see water except for the occasional rinsing and drying. Those get lightweight honing oil or tapping oil. Similarly, I'll never use oil on my water stones - since I use them with water, soaking them with oil will cause them (eventually) to not accept water, and I'll lose grinding efficiency and it will either take longer to keep the same edge, or I'll end up with a stone that wrecks knives. Neither is desirable, and both can be traced to "a different additive" - even though both additives would be to accomplish the same ends. Make sense now?

5-90

 

[75SV1]

The mixing of synthetics and dino or not to is carried over from the old days. When synthetics came onto the market in the mid to late 70's, you couldn't mix them with dino oil. In the early to mid 80's you could. I've switched back and forth a few times on an 96 Ford Contour, with no problems. But, I've used full synthetics for 145K of 170K. The engine doesn't use much if any oil in 5-9K changes, maybe 1/2 a quart. You should still be able to find some synthetic blends. NAPA sell Shaffers oil. THey make syn-dino blends. Suppose to be excellent oils.
Tom

 

[yellowxj]

My turn for .o2 ...
I've been mixing oils for 5 years. Why? Just cause it makes me feel better. I own a used car lot where I sell cars "on time" (by here pay here) to the most auto maintenence challenged people in a first world country. A portion of the cars I sell wont have their oil changed again. People are ignorant. 28k is the current car lot record for confirmed lack of oil changes. I've had very few of them manage to blow up motors and none of them were caused by oil not working. Dont know if my mixing oils based on how what I feel the engine needs makes it work or not...

I've heard the difference in a 4.0 on 10-30 and then on 20-50...I've run even straight 30 or 40 in my jeeps before too. I always use the discount auto/advance auto parts motor oil too...works fine.

The 4.0 can handle all of them with out problems and if you keep running that 10-30 water in a old fashioned iron motor it will wear out quicker IN MY OPINION. You can also put too thick oil in some motors...

4.6L ford motors have a hydraulic timing belt tensioner (IIRC) that, when you us too thick oil, will put too much tension on the belts and stretch them so that if you do go back to thin oil they will be loose. Some kia's dont like thicker oil, lifters start tapping (that may have to do with sitting around at the car lot too, either way when you drive them for a long time the tick goes away/the oil gets hot and thins out a hair...nobody takes care of their kias :explosion:

4.0 has the loose enough tolerances to handle most weights.

 

[75SV1]

I use to run 20W-50 in a Ford 351W. I noticed a 2 mpg lose from running a 10W-40 or 10W-30. You are right, some engines run bad on thicker weights of oil. My Ford Contour does better on 5W-30 than 20W-50 synthetics. Runs much smoother. I ran 15W-40 Shell Rotella in the 4.0L. I alway had a tick in one of the lifters. It might have been poor maintanace from the previous owner.
Tom

 

[Matthew Currie]

My turn for .o2 ...
I've been mixing oils for 5 years. Why? Just cause it makes me feel better. I own a used car lot where I sell cars "on time" (by here pay here) to the most auto maintenence challenged people in a first world country. A portion of the cars I sell wont have their oil changed again. People are ignorant. 28k is the current car lot record for confirmed lack of oil changes. I've had very few of them manage to blow up motors and none of them were caused by oil not working. Dont know if my mixing oils based on how what I feel the engine needs makes it work or not...

I've heard the difference in a 4.0 on 10-30 and then on 20-50...I've run even straight 30 or 40 in my jeeps before too. I always use the discount auto/advance auto parts motor oil too...works fine.

The 4.0 can handle all of them with out problems and if you keep running that 10-30 water in a old fashioned iron motor it will wear out quicker IN MY OPINION. You can also put too thick oil in some motors...

4.6L ford motors have a hydraulic timing belt tensioner (IIRC) that, when you us too thick oil, will put too much tension on the belts and stretch them so that if you do go back to thin oil they will be loose. Some kia's dont like thicker oil, lifters start tapping (that may have to do with sitting around at the car lot too, either way when you drive them for a long time the tick goes away/the oil gets hot and thins out a hair...nobody takes care of their kias :explosion:

4.0 has the loose enough tolerances to handle most weights.

I would add a little comment for those of us not in your climate. I have no problem mixing oils either, but here in Vermont a 4.0, a Chevy 350, or other good ol' cast-arn mill will happily run for ever on 10-30 or even 5-30 year round unless there's already something wrong with it. Even in summer we have cool mornings, so I prefer a pretty thin oil for start up lubrication. I don't worry as long as the oil pressure stays within range. If it's there it's there, and added thickness, while it makes for a more reassuring gauge reading, doesn't perform any useful work. I've always been of the opinion that if your bearing clearances are correct, then you do better to maximize flow rate rather than pressure, since one of the most important things oil does is transfer heat.

 

[brianb2247]

drop the oil pan and replace the pickup for the oil pump, the screen gets clogged. very common