What does everyone use when changing the diff fluid? I know ford says 70w140 syn but in f150net the write up says 75w90 syn. I was thinking of changing mine this weekend, its a 98 with 86k miles on it. I dont know if its necassary to change it or not but I was going to do it for preventative maintanence.

 

I believe that it is 75W140. I just did mine in the fall. You have to use blue or black silicone gasket maker. I fopund this diff oil to be VERY expensive so I ordered Royal Purple online which was less than half of the cost and in my opinion, much better gear oil!
GOOD LUCK!

 

Ford used to recommend 75w90. They changed their specs a year or 2 ago. I had mine changed due to a shudder problem and the rplacement was 75w140, even though the original specs in the owners manual states 75w90. Also make sure if you change it and havs a limited slip, you add the friction modifier.

 

I dont know anyhting about the friction modifier. I was changing it with synthetic oil. Its a stock 98, dont know if its limited slip or not.

 

Look at the door code for the rear end. If it is 2 numbers, ex "19", it is an open diff. If it is a letter and a number, ex "A9", it is a limited slip. You do want to use the friction modifer if it is limited slip, even though the gear oil is synthetic. Assuming it is already in the gear oil. If you don't, you will get a shudder when pulling out on a turn.

 

Royal Purple 75W140 is fully synthetic with everythign added that you need whether you have a limited or non limited slip rear end, it says so right ont he bottle!

GOOD LUCK

Brian