I have a 99 F150...4.2, auto, 171000 miles....I want to service the differential, can I or should I use a synthentic lube? If so what are the specs?
Thanks for any suggestions..........

 

75W 140 synthetic. If the rear end is limited slip, don't forget to add the friction modifier. Look in your owners manual, it should tell you everything.

 

Unfortunately, the owners manual does not usually mention the friction modifier, and back then the recommended was 75W90. Since changed to the 75W140.

 

I recently did gear oil changes on my differentials. I used Mobil 1 Synthetic 75-W140 in the rear, and have not added any friction modifier. The Mobil 1 website states you can add it if you want, but it's not necessary: https://www.mobiloil.com/USA-English...75W-140LS.aspx

After putting about 200 miles on the truck since, I have not had any problems without the friction modifier.

FYI I used Mobil 1 Synthetic 75-W90 in the front and Mobil 1 Synthetic ATF in the transfer case.

 

Good Info....I use Mobil 1 syn in the engine....might as well stay with mobile1