Jeep 4.0 "perfers" 40 weights

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I've done alot of searching concerning the Jeep 4.0 engine. Alot of people are recomending 40 weight oils. Why? How many of you have tried this with good results? Is there a gas mile improvment? Or are the UOA's coming back with better specs? Thanks in advance for you replies
 
Hopefully you are not talking about straight 40 wt oils, but a xW-40. My 93 GCL 4.0 never saw anything less than a 15W-40 from the time I bought it (1999 with 55,000 miles) til I sold it in 2002 with 85,000 miles). I started with 15W-40, then used 20W-50 to quiet the lifters for a year or more until I could bring new lifters in my suitcase (hadn't heard of Auto Rx). Then I went to Delo 15W-40 and it ran fine. Never added a drop between 4,000 mile changes. Oil presures at 0 C were little lower (the guage almost pegged on 20W-50). This Jeep never saw normal driving. I normally flew to were I kept it and traveled to the mines and outback in it, mostly at 14,000 ft altitude.
 
Yes, I'm refering to xW40 oils. Alot of post reccomend it ,but haven't seen any explaning the benifit thanks again for you reply
 
My 95 4.0 GC perfers 5w-30. If i use a heavier 10w-30 (Castrol GTX, or Chevron) my oil pressure guage is pegged at 80 psi. With 5w-30 Mobil 1, my pressures are 65 PSI at highway speeds, and 35 PSI at hot idle.
 
"I've done alot of searching concerning the Jeep 4.0 engine. A lot of people are recommending 40 weight oils. Why? How many of you have tried this with good results? Is there a mileage improvment? Or are the UOA's coming back with better specs?"

A&PTECH, try searching in the UOAs forum here for gas engines. Try "4.0" and/or "Jeep" and you should get plenty of hits. There have been at least several posted here.

--- Bror Jace
 
My new 95 Jeep 4.0 was changed over to Mobil 1 at the first 3000 miles oil change. I used Mobil1 5w30 below 30 degrees F. and 10w30 summer. I sold vehicle at 150,000 + miles with no leaks and using about 1/2 quart of oil on a 5,000 mile change interval. 40 weight will kill any hope of obtaining a high millage engine unless you travel coast to coast and seldom shut it off. ed
 
"40 weight will kill any hope of obtaining a high mileage engine unless you travel coast to coast and seldom shut it off."

Um Eddie? I believe there are Jeep owners here that have experiences which will contradict that assertion.

We'll probably hear from them soon.
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--- Bror Jace
 
If anything a 40 weight will slightly hurt gas mileage, but it's probably not even a measurable amount. My cherokee has almost 200,000 miles, and has had 10w30 all it's life. The only thing ever replaced was the cat, and it doesn't burn or leak a drop of oil. Really, any oil will work well in this engine as long as it's kept clean. You could probably use bacon grease and be fine.
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Why is 40 weight better for this engine? Because it's design has barely changed since it was introduced in the '60s, and hasn't changed at all since 1987. It's just a big old hunk of iron, and doesn't have very tight clearances...
 
quote:

Originally posted by JonS:
My 95 4.0 GC perfers 5w-30. If i use a heavier 10w-30 (Castrol GTX, or Chevron) my oil pressure guage is pegged at 80 psi. With 5w-30 Mobil 1, my pressures are 65 PSI at highway speeds, and 35 PSI at hot idle.

Change your oil pressure sensor and you will be suprised at how off it is. Mine was reading the same..... I changed it...... well let just say i was freaking out for no reason.
 
I agree with your finding and opinions OmOZ as in my previous experience with the 4.0 engine. In addition my son-in-law is a Jeep technically service rep. and he also recomends a good grade 10w30 until you get around 200,000 miles and then go to 15w40 or 15w 50. He just changed to 15w50 last year at ~215,000 miles.
 
my friend's 95 Cherokee has 225k miles on it, all of them on conventional 10w40. Still runs pretty well.

--Matt
 
Amsoil ATM 10w-30 does great in this engine. It's a thick 30wt oil.
 
quote:

Originally posted by JeepZJ4.0:

quote:

Originally posted by JonS:
My 95 4.0 GC perfers 5w-30. If i use a heavier 10w-30 (Castrol GTX, or Chevron) my oil pressure guage is pegged at 80 psi. With 5w-30 Mobil 1, my pressures are 65 PSI at highway speeds, and 35 PSI at hot idle.

Change your oil pressure sensor and you will be suprised at how off it is. Mine was reading the same..... I changed it...... well let just say i was freaking out for no reason.


This is with a new pressure sensor. I had it changed a year ago because it was reading low (40 PSI) at speed and 20 PSI at idle.
 
quote:

Originally posted by JonS:

quote:

Originally posted by JeepZJ4.0:

quote:

Originally posted by JonS:
My 95 4.0 GC perfers 5w-30. If i use a heavier 10w-30 (Castrol GTX, or Chevron) my oil pressure guage is pegged at 80 psi. With 5w-30 Mobil 1, my pressures are 65 PSI at highway speeds, and 35 PSI at hot idle.

Change your oil pressure sensor and you will be suprised at how off it is. Mine was reading the same..... I changed it...... well let just say i was freaking out for no reason.


This is with a new pressure sensor. I had it changed a year ago because it was reading low (40 PSI) at speed and 20 PSI at idle.


Actually, that is the correct pressure..... according to the manual.... 20 PSI at idle is good (15 is the min) 60 is the max PSI at speed (40 is good)
 
Yeah, but 40 is on the low side of recommended pressures. That is why i almost went with a 10w-40 or 15w-40 which i felt would be band-aiding the problem. I e-mailed Havoline a while ago about this and their response was that a 10w-40 might not raise the pressure because a 10w-40 or 15w-40 would be thicker and the added drag of the heavy oil would make the oil hotter and i would end up with the same pressure. If my 30w oil temps ran at 180, 40w would run at 200 and be the same vis as a 30w. That's why i like to run thinner oils. They flow better and faster.
 
quote:

Originally posted by JonS:
If my 30w oil temps ran at 180, 40w would run at 200 and be the same vis as a 30w. That's why i like to run thinner oils. They flow better and faster.

All else being equal, the 40 weight will run hotter but not hot enough to be as thin as a 30 weight. If it did for some reason get hot enough (like 200F) to be as thin as a 30 weight at 180F, then the viscous losses would be as low as the 30 weight at 180 F so it would cool off because the extra shearing wouldn't be generating enough heat to hold it at 200F.
 
I dunno Jon... not to start a fight or anything but i know 20psi at speed is the low side. 40 is right in the middle. Mine runs about 45psi with 10W30 oil.
 
quote:

Originally posted by JeepZJ4.0:
I dunno Jon... not to start a fight or anything but i know 20psi at speed is the low side. 40 is right in the middle. Mine runs about 45psi with 10W30 oil.

Is that actual oil pressure off an accurate gauge, or from the Jeep gauge?
 
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