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Full Synthetic Oil in the 4.0L?

Q

QuillsXJ

NAXJA Forum User
Location
California
I was thinking about making the switch from my typical Chevron Supreme 10W-30 to a full synthetic equivalent for the remainder of the life of my engine. What do people think of running full synthetic in this engine? Does it seem to help the engine run better or has anybody noticed that their oil stays cleaner longer? I have a 92 XJ with 209,000 miles and it does not leak oil.
 
I run Mobil 1 or the Shell Rotella synthetic. No issues. I do 5000 between changes and use a larger oil filter. I use 5w30 Mobil in the winter and the shell in the summer. Temps here can run easily to -20 in the dead of winter and can run into the 90's during summer.
 
When I started using Mobil 1, I noticed a difference immediately. Cold starts were much easier and it seemed to run smoother overall. I also used the high mileage synthetic, and it seemed to help with the minor leak I had. I'm going to switch to regular 5w30 non high mile on my next change. I also use the larger oil filter which is a Motorcraft filter, because apparently it is made by Purolator, which is good.
 
I run 100% synthetic in my '97 4.0L, but there is less than 12k on the engine and it gets a good workout being a wheeling only rig. My '99 XJ with ~159k on it gets whatever good semi-synthetic is on sale at the time, usually the $20 special. Nothing has been done to that other than I re-did the top end at ~130k.

For an engine with 209k+, I wouldn't bother with synthetics. Run whatever cheap dino oil you are comfortable using. With that kind of mileage on a tired engine you aren't going to gain much other than a lighter wallet.

The larger filter on the other hand is a good idea, just make sure you add extra oil.

Stock Filter:
Mobil: M1-204
Purolator: PL14670
K&N: HP-2004
Wix: 51085

Larger Filter:
Mobil: M1-301
Purolator: PL30001
K&N: HP-3001
Wix: 51515
 
Last edited:
I've got around 200k and I currently run Amsoil AMO (10w-40) full syn / Napa Gold Filter (same as Wix) with no issues. Mileage increased a bit when I switched, seems to run better too. I usually go about 7.5k between changes so cost is about the same as if I were to use dyno oil and do changes more often.

If you have fluid leaks they may show more with the syn. What I have done in the past with higher mileage engines is run a syn-blend first. Then take the next step.
 
I run Mobil 1 full synthetic high mileage in my 98 XJ with 150k on it. I know our oil pressure gauges aren't the most reliable units in the world, but the gauge did indicate a bump in pressure of around 1-3 psi, and it seems to run a bit smoother. I have not noticed any new leaks and have been running this with a Mopar filter for around 5,000 miles.
 
I have been running synthetic since 55,000 miles when I bought it. I now have 181,000.

Switched to Amsoil 3 years back when I became a dealer.

I pulled the oil pan a year and a half ago to do a rear main seal and the engine was very, very clean and I have no issues. Great oil pressure, etc.

Normally I do 10,000 mile changes, but I am getting ready to change it this weekend after 20,000. I expect no issues.
 
RCman said:
I run 100% synthetic in my '97 4.0L, but there is less than 12k on the engine and it gets a good workout being a wheeling only rig. My '99 XJ with ~159k on it gets whatever good semi-synthetic is on sale at the time, usually the $20 special. Nothing has been done to that other than I re-did the top end at ~130k.

For an engine with 209k+, I wouldn't bother with synthetics. Run whatever cheap dino oil you are comfortable using. With that kind of mileage on a tired engine you aren't going to gain much other than a lighter wallet.

The larger filter on the other hand is a good idea, just make sure you add extra oil.

Stock Filter:
Mobil: M1-204
Purolator: PL14670
K&N: HP-2004
Wix: 51085

Larger Filter:
Mobil: M1-301
Purolator: PL30001
K&N: HP-3001
Wix: 51515
Click to expand...

Add Motorcraft FL1A to the large filter list.
 
Mobil 5-30 full synthetic since about 65K. Minor valve cover leak started right after the switch, but, hey it's a Jeep 4.0. I think the valve cover's are supposed to leak, aren't they?
 
el_roy1985 said:
When I started using Mobil 1, I noticed a difference immediately. Cold starts were much easier and it seemed to run smoother overall. I also used the high mileage synthetic, and it seemed to help with the minor leak I had. I'm going to switch to regular 5w30 non high mile on my next change. I also use the larger oil filter which is a Motorcraft filter, because apparently it is made by Purolator, which is good.
Click to expand...

Forgot to mention that I switched at about 150k. Even with a higher mileage engine, it doesn't hurt to switch. It will still prolong the life of your engine and be a lot easier on it during extremely cold starts.

I personally tested conventional and synthetic oils in a pour test before. It was a below zero day out, so I left a bottle of each outside in the snow for a few hours. Then when I went to pour them, you could tell a major difference in how thick they had gotten. The synthetic still poured rather freely, while the conventional was noticeably more sluggish. That would be enough right there to switch me over had I not already been running mobil 1. I actually did it to convert a fellow worker to start using Mobil 1 in his car, instead of the Havoline he was used to.
 
To me running CLEAN in older engines, particularly those that may not be DD or see a lot of cold start/short trips is very important.

I run Shell Rotella 10w30 Dino. I end up changing my oil more or less on a 3 month schedule rather than from mileage, and I do a lot of cold start/short runs with the occasional abusive weekend in the mountains. Winters in my part of So. OryGun are mild so I don't even bother to change viscosity.

Some really good advice from everybody so far, so make your decision of what is best in your locale and vehicle usage.

Good luck.
 
My DD '98 Cherokee has had 10W30 Mobil 1 in it since new. It now has 200,000 Kms on it. My last ZJ 4.0L went to 300,000 KMs on Mobil 1 before I sold it. The benefits of synthetic lubricants are well documented and are no different for a Jeep 4.0 than any other engine.
 
I run the Amsoil 10W30 with Amsoil EA042 filter. I currently have 173,000 miles on my 2000 XJ with no problems. Runs like new.
 
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