Why does Jeep 'like' 40 wgt?

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Jul 10, 2003
Not Seattle, but close.
I've read several posts saying the 4.0 likes a 40 weight oil. What is this based on? I know it's an older design, but I've followed the owner's manual and used 10W30. What's the deal?
They seem to be built looser and the 40 weight seems to help hot idle oil pressure and quiet down piston rocking when the motor is cold. They just seem to "feel" better with the 40 weight or a heavy 30 weight oil.
Jeepers that come on to this board have to realize that running a good quality 10W-30 in the 2.5 or the 4.0 litre engines and doing timely oil/filter changes will result in 2-300,000 miles on these motors.Here at Bob's we seek a higher lube power and go by oil tests rather than "my father used it so will I"....thats all...use what you like this site is a great reference point to start from.I use Delvac 1 5W-40 in both my Jeeps...thats just me [Big Grin]
Originally posted by MarkC: Maybe I'll just try one of the heavier 10W30's.
Citgo Ultralife (high mileage) 10w30 lists a 100C viscosity of 12 cSt. Compare Maxlife at 10.5 cSt.
I sold my 95 Jeep 4L at 151,000 miles. Used about 1/2 quate make up on a 5,000 mile oil/filter change. Used Mobil1 10w30 and PureOne oil filter all its life. 40 weight I would not used unless the engine is very worn due to miles or neglect.
Just speaking from my own experience with 2 different Jeep 4.0's, I've run M1 10W30 since they were new and both run perfect (96 with 130K, 2000 with 62K). I'm doing my first UOA on both over the next couple of weeks so the results should be interesting. Both have excellent oil pressure (I don't remember the numbers right now).
Originally posted by TallPaul:
Originally posted by MarkC: Maybe I'll just try one of the heavier 10W30's.
Citgo Ultralife (high mileage) 10w30 lists a 100C viscosity of 12 cSt.

Thats a 10k mile oil in the right engines/driving conditions [Wink]
I used to use a 30w-70 in a 258/4.2 years ago and it worked just fine. [Big Grin] Prior to that it was a steady diet of 20w-50, in both the 232 cu. in./3.8 litre and the 4.2 litre. The 30w-70 only went in when I sheared down a 20w-50 Valvoline XLD on a 44*C/112*F day. It lasted for years on that stuff, and would have had over 300 000 miles when it was retired and only blew a small puff of oil smoke on start up. (valve guides) Of course I wouldn't run that grade these days. [Big Grin] Rick.
why would use of a 40 wt oil decrease mpg or hinder the performance of a jeep engine? I have heard the positives that it "quiets" down a jeep but my jeep doesn't have a ping.
I use M1 10w-30 in my 95 4.0. The hot idle is 35 psi, and at highway speeds it is 60 psi. I think a 40w is too thick for this engine. Thinner flows better, keeping the engine and oil cooler.
Many of you who quote the owners manual as only requiring 10w30 have manuals from new model years. The manual for my '88 Cherokee with the 4.0 allows the use of all sorts of weights of oil from 5w20 (in temps that remain below zero) to 20w50 (temps stay above 32F). Certianly there is no requirement that the engine only run on 10w30, as we all know the changes made in later years were minor. BTW, the suggested service interval on SF dino oil was 7,500 miles. [Razz] In my case, I've run dino 5w30 and 10w30, and I'm now running Rotella T Syn 5w40. In terms of oil pressure, at idle when hot on any, I'm running 25 psi. When running hot at speed, the dino 5 and 10w30's begin at 60 psi, but all fade back to near 45 psi when run for more than an hour. The 5w40 stays right at 60 psi, even during an 8 hour run in 95 degree weather. My 1 UOA hasn't borne out that 5w40 is better, but I do like the maintained oil pressure. Another UOA is due in less than a month... And lest anyone think I'm a thicjer is better kook, just remember I ran 6000 mile OCI's on 5w20 in my '99 F50 I switched over at 80+k!
Originally posted by MNgopher: The 5w40 stays right at 60 psi, even during an 8 hour run in 95 degree weather.
The mid 90s Jeep oil pressure spec for the 4.0 is 37 psi @ 1600 rpm. Don't know what the '88 spec is, but perhaps you were fine with a 10w30.
I have tried to find this on this site and the web but cannot get a answer I fully understand. I have seen UOA and have seen that rotella 5w40 may protect better then M1. However, I have heard some people say don't use a 40wt unless your jeep engine is loose and has hig miles. What makes a 5w40 different from a 5w30? I would love to switch to Rotella 5w40 because It is a lot cheaper than using m1.
wn1998 Some have found that a 5w-40 gives better wear numbers than xw-30. Not necessarily that a particular 5w-40 is better than "any" M1. If it is somewhat proven that the reduced wear is due to the heavier weight ..it would be just as good for M1 in a heavier weight (0-40 perhaps) ..MAYBE. Since we can't just run lube tests continuously 24/7 ..and since many, like myself, are into extended OCI (or don't accumulate enough mileage in an appropriate amount of time = time weighted OCI), you have very limited data points in which to draw conclusions. Me? I did a 9k UOA on M1 xw-30. I did a 10k UOA on Delvac 1 ..wear was down substantially. Is it the heavier weight "alone" or some other feature of Delvac 1 that was responsible? In the case of Rotella T synth ..then costs come into the picture. If I can get the same results with Rotella T @ $3/quart vs. Delvac 1 @ about $6 or if I can get almost as good results over the same duration ...or the same results over half the duration ..yadayada ..See what I mean [I dont know] Since I'm commited to synthetics as a rule ..and I've found a product that fits my intened use ..I'm naturally going to give you advice based on my slant on things. So far there's not enough data on users comparing different oil weights in "the same engine" to pinpoint any one aspect of reduced wear. The available data suggests that heavier oils work well with it. In my case under extended OCI. FWIW: I'm due very soon for my 12.5k OCI on my wife's Sahara. I'm going with Rotella T synth and will go for 15k this time. It will take about 9 or 10 months to accumulate the mileage. Life is good [Smile]
Yep I am not convinced it needs or likes 40wt. There is another analysis too. It had M1 0w-40 and still high iron. Next 0w-40 and LC and voila, not high anymore. All of the 40wts that have reduced the iron have been deisel oils, I think the key is there, not in the visc, as the M1 analysis proves as well...
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