What is the best oil to use for the rear diff on the 99 F150XL 4x2 with a 4.6L??
Any suggestions?

 

i just ran 80-90 wt this time since i had it on hand after rebuilding my rear diff fully for the second time. pinion sheared a few teeth somehow. never heard the "techs" opinion.
just be sure to use friction modifier, i read and hear it does wonders in these axles

 

Where do I get this friction modifier?
Is this a dealer item?
Do regular parts stores carry it?
I was told the rear ends needed a different oil in them, but if 89w90 is good then that is easier.
Thanks.

 

75W140 with the friction modifier. You want the dealer brand Friction Modifier, the junk at autozone in the tube is just that, JUNK!. I only use Motorcraft which is $17 a qaurt!!! In the Camaro I use Royal Purple, little cheaper

 

im not really sure if the 75-140 is that much better than 80-90 lol, to me that thick of oil in the rear just dont matter much anymore. plus as many times as i was in there, it didnt seem to matter :/

i was using checkers/schucks line of friction modifier. but then i never really did find out what the hype is about it. my truck bounced down the road since i got it lol

 

Unless you have a limited slip differential, the friction modifier is unecessary.

 

I was just about to say this.
Put some Amsoil in your rear diff. Rear limited slip/ controled slip rear dif. = Severe Gear 75w- 140 (SVOQTC)

 

The rear axle in a 99 *requires* a full synthetic 75w140 unless you have aftermarket gears in it, then you follow the gear manufacturer's recommendations. If you have the factory limited slip, you also need a bottle of the Ford friction modifier unless you use gear lube that already has it incorporated.

 

Well I think I will try the dealer for the 75-W140.
I will also see if they have the friction modifier.
I might as well play it safe on this one.

 

Get the modifier from the dealer, but you can use any 100% synthetic 75W140. I believe Royal Purple already has modifier.

While you are at the dealer, also get a tube of the silicone sealer for the cover.

 

eh? why silicone from the dealer? whats wrong with regular parts store silicone? like the right stuff or whatever?

 

Why would I not get the proper diff gasket instead of silicone?

 

most places dont carry them anymore.

 

Yeah the local parts store around here have them.
I was going to get one the other day, and decided to wait to see what kind of fluid I needed first.
The gasket was only $10.00 and was a FEL-PRO so I will go get it tomorrow.
Thanks again.

 

They don't use a gasket at the factory, they use specific sealer.

Some generic sealers have a tendency to leak, that's why I said the dealer to be totally safe. There's a thread around here somewhere with the exact part number and the factory procedure for using it. If you can get a gasket and want to use it, fine.