I have a 1997 F-150 ex cab 271K miles with the 4.6 and i changed my rear end fluid with valvoline syn blend 80-90 plus the limited slip additive. Thats what they said it called for, but i see on here that it calls for 75-140? Is it going to kill it to run 80-90? thanks! adam

 

Who is "they"?

What does your owner's manual call for? If you don't have one, you can download it.

http://www.fleet.ford.com/maintenanc...ls/default.asp

 

Im sorry haha. The lady at my local advance auto looked in her computer and she said 80-90. I called to my local ford dealer and they said 80-90 wont hurt anything but by the book 75-140 is what it calls for. Thanks anyways guys!

 

i think i put amsoil 90-140 maybe, in mine

 

I just did the research on this as I changed out my rear diff cover and fluid.

For the front diff it is 75W-90 GL-5 synthetic gear oil.
For the rear diff it is 75W-140 GL-5 synthetic gear oil.

It does not matter if your rear diff is an 8.8 or a 9.75. If you have an LS rear diff add 1 4oz bottle of friction modifier (from Ford). Unless you change out your rear cover to a larger one as I did.

 

Ford Came out with the 75w140 viscosity because they were losing so many differentials due to low quality products being used. To get that wide spec fluid will require the use of synthetics. If you used a high quality synthetic 80w90 it should be fine. Using a conventional gear lube would not hold up to the heat and shear caused by these trucks.

If you do lots of towing, I would probably go for the 75w140 product, if you don't tow, the 80w90 would be fine