Why is 10w30 "preferred" for jeep I-6?

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42
Location
Oregon
The manual says that 10w30 is preferred and to use 5w30 if you experience sub zero temps. I'm assuming because the oil clings to the parts a little better. My instincts tell me to take it down to 5w so I can get better flow at startup but I can't help but think the engineers chose 10w for a specific reason. I know lots of people run 5w40 and 15w40 in this engine as well so if anyone could give me some insight that would be great. I've already decided on rotella with a pureone filter for my next oil change all I have to do now is settle on a viscosity.
 
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4,220
Location
Massachusetts
Typically, the 10W30 lubricants of yesteryear were more robust than their 5W30 counterparts, which could shear down into the 20wt viscosity range. Fast forward to the present and 5W30 conventional lubes have drastically improved. Being a Jeep guy and having rebuilt many Jeep engines, a modern 5W30 conventional should serve you well. I recommend SOPUS products, as they are quality lubes at a great price. Many here on BITOG have posted UOA info on various oils used in the Jeep/AMC 4.0L. Typically, this design is known to shed some iron. Heavy 30wt or 40wt oils usually produce the lowest Fe numbers. In my experience, I have run SAE30, 10W30, & 10W40 all with great numbers, but for the last several years, I have run Quaker State GRPIII synthetic 5W30 in my trail Jeep's 2.5L. I have obtained similar Fe numbers running this oil and appreciate the quicker revs and lower parasitic drag compared to heavier oils.
 
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5,170
Location
San Francisco Bay Area
Same was true for GM 3800 Series II engines from the 90s. The 5W-30 oils at the time were probably more volatile. 20 years later and the API SN standard is easily better.
 
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7,462
Location
Mahzurrah!
The Jeep 4.0 will seem to run fine on a 5w30 but if you do a UOA and it has sheared it won't be pretty. It needs a good 30 grade, thus the 10w30 recommendation and frankly a thick 30 is even better, like Rotella or Defy. If you must run 5w30 use Valvoline, multiple UOA's in the 4.0 have shown it to have above average shear resistance and it produces excellent "wear" numbers in the 4.0 too.
 
Messages
842
Location
VA
The 4.0L runs best on a thicker 10w-30 such as QS Defy 10w-30 , Rotella T5 . Other good lubes for this engine are PYB or VWB . I run Defy 10w-30 and am very happy with the results.
 
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3,182
Location
Ohio
Originally Posted By: y_p_w
Same was true for GM 3800 Series II engines from the 90s. The 5W-30 oils at the time were probably more volatile. 20 years later and the API SN standard is easily better.
Incidentally the 87-89 3800's SI recommended 5w30 until the "Tuned port" 3800 came out in the Regal GS in 1990. The 3300 from 89-90 also suggested 5w30 and switched to 10w30 for 91-93. In 2003 the 3800s suggested 5w30 again. Don't ask why I know this worthless info, lol. As for the O.P. I think the Jeep 4L was in the same boat as the 3800 as far as the 10w30 suggestion. During that time period the 5w30s weren't holding up is my guess. Having said that....I ran 95% of the time 5w30 conventional/blends/full syn in my 98 Regal for 6 years. That engine (which is in my garage) has 221,xxx original miles on it and has seen about 75 drag strip passes (not counting the multiple street floggings). 5w30 should serve you well. That's what we put in them at work. However there are some evidence that maybe a HDEO would be a better choice for the 4L.
 
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7,462
Location
Mahzurrah!
That's the problem with the 5w30's, they run great you think it's fine but if you do a UOA you'll often find you could've done much better. A 4.0 will usually shear a typical 5w30 with a [email protected] of 10.5 down to about 9 in only 3,000 miles. At that point it's too thin for optimum protection, the 4.0 needs thicker lube than that. It will also do the same thing to lesser 10w30's, so if running a standard thickness 10w30 a top quality name brand needs to be the choice. Valvoline has been the stand out in this bracket Maxlife or White bottle no matter. Better yet though is a quality 10w30 on the thick side, like Rotella or Defy. Ironically I can count the "good" UOA's I've seen for the 4.0 using top tier synthetics on the fingers of one hand with some left over. The 4.0 thrives on a good dino or at most a syn blend like Maxlife or T5.
 
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