Which axle? I have a '99 F150 with an axle leak. The cover of the housing is leaking and needs to be replaced. I have the new cover and 75/140 synthetic. I have the axle with the 9.35 ring gear. None of the tags have any of the codes that were in the FAQ's on this subject. This isn't the original axle on the truck. My question is, do I have a L/S axle that needs the friction modifier additive?

 

Look at your door jam sticker under axle and there will either be a letter and a number or two numbers. Here is a link to the door sticker decoder hope it helps you out.
https://www.ford-trucks.com/article/...r_Decoder.html

 

If you have the 5.4L, then you have the 9.75" rear axle

 

Does the rear end still have a tag bolted to the cover bolt? It should have the code on it. Usually on the bottom line, the first few digits have the axle ratio. Something like 3 55 9 75 would be a 9.75" gear 3.55 open. It may also have 3L55 which would be 3.55 LS and would need the additive.

 

Thanks Lx. Looks like the 3.73 L/S.