I have heard some theories on this with 75-90 VS 75-140. I am about to swap break in oil with a synthetic 75-140 for LS. I have a tracloc, with yukon 4.10 set.
Anybody think going with the 75-90 dino, with a slip additive as the service manual prescribes is better, and if so why. I can't see why I would want that in it, but figured I would see if you guys knew of a great reason to do it that way.

 

Automatic Transmission Fluid has friction modifier additives which changes the friction of the clutches. Same thing with "traclock," a manu. specific term for limited slip differential. Yes, you must add the friction modifier because it has clutches and if you don't it's probably going to slip excessively or slip too much. Follow manufacturers recommendations, always.

 

Yes I was aware of all that, thanks for sharing with me though. My question isn't about friction modifier I probably should have worded my question different, but about the weight of the gear oil itself. Problem is a few contradicting TSB came out from ford on the 8.8 regarding favoring the 75-140 over the factory fill. A drag shop in Ohio i purchased the clutched up tracloc unit recommended 75-140, but disclaimered it with telling me factory was 75-90 in 1989 based off the book..

I am probably putting in 75-140, just wanted to see if anybody has some unique insight into this question.

 

If ford put out a TSB stating to use 75-140, use that. The diff is likely "tighter" with a higher viscosity fluid making the clutches last longer, is my guess. Just like on fords, 4.9, 5.0, and 5.8, ford put out a tsb stating that 5w30 in the engines will further increase fuel efficiency and reducing engine wear over the original spec of 10W30. It's more than likely a superceded bulletin, replacing the old spec.

 

Do you know what years the TSB related to the 302 spanned? I run 10w30 valvoline( napa gold filter ). I swear by those napa gold filters, never found anything that cleans as well.

 

I use 5w-30 in my '90 and never had any issue.

 

Dude, the napa gold filter is a WIX rebranded, haha. WIX are descent quality, stick with it. Good silicone drainback valve. That would take a lot of digging around to find out.

5W-30 is a slightly thinner 5 weight oil that can get into tighter clearances on start up but remain a good 30 weight at 212°F. That is why some newer tighter clearance engines use 5W-20 instead of 10W-30.

 

Yes I knew they were wix, i learned that a couple years back when i was trying to find one for my farm tractor, and they ordered one for it, it came as a wix, not a napa gold.