I just inherited a 2004 F150 XL from my late dad. Anyway I noticed that the rear diff is dripping at the diff cover. The truck has a 3:55 limited slip rearend. I called the loca Ford garage to see about fluid's, and gasket. Now the owners manual and the official Ford workshop manual both say to use 75w-90 full synthetic with 4oz. of friction additive in the rear. The Ford garage told me I should use 75w-140 in th rear. They also say there was no diff cover gasket but sillicone was used. Now 75w-140 cost more than the 75w-90. Is what Ford told me true? should I use the 75w-140? And I don't know a whole lot about lube weights so what is the differance between the 90 and 140 weight?
Thank you for any info you can give me.

 

You need to use synthetic 75W140 and a bottle of Ford limited slip additive. If you would prefer to use a gasket, they are available at Autozone for the 8.8" rear, I don't know about the 9.75" rear. If you do NOT have a 5.4 engine, you have an 8.8.

 

If you use RTV cement let it set up befor fill of axle. This insures no leak. Also streighten up th indents around the bolt holes in the cover by using flat surface and hammer. This will insure a steady pull on the sealing surfaces. If you live in a milder climate def use 140 while if you are northern use 90 for better winter usage.

 

75w90 was the original specification. Ford has since changed to the 75w140 for all F150 rear diffs. That is why the conflict. Go with the 75w140 and the friction modifier.

 

75W140 is the current recommendation for ALL temperatures. The Ford garage is correct.