7.3 f350 what rear diff. fluid?
 

What is used in the rear diffs. of these? just a 80/90 weight gear oil or?

I just changed mine tonight in my f25o with 75W 140 synthetic. Takes 3.3 Liters

That's the weight of oil that I used as well.

Is it necessary to use the synthetic for the limited slip or something like that? Im just wondering, I plan on using the synthetic anyhow. That is the correct weight to use though?

Absolutely must use syn. Ford manual specs syn 75W-140 plus friction modifier
for limited slip systems.
Nut

yes, 75w-140 is for the rear, but 80w-90 is for the FRONT.

I am changing out mine as well as adding the Mag-Hytec rear diff cover and had this question in the Super Duty section of FTE. Someone over there said this: "Just to let you all know, I called another local dealership today and spoke with their parts department. A TSB has in fact been sent out by Ford stating that the front axle 75w90 Gear Oil should be replaced with 75w140 when doing a fluid change. The TSB numbers are 04-24-20 and 05-12-06. Hope this helps."
The rear is supposed to be 75W140 anyway I think?
I also saw in that thread that some of the synthetics already have the friction modifier in them for my limited slip and some don't???? Which brand are you guys using (Amsoil, Royal Purple, other??)

I put Royal Purple in mine, 75W140. Royal Purple states that there is no need for a friction modifier with their gear lubes. Also, their oil is actually PURPLE, I thought it was just name sake.

Valvoline syn power is the only 75/140 I can find on advanced auto web site they have others that are 85/140(Lucas) and others. is this other safe to use or what should I get? thanks~!

I use OEM Motorcraft with the additive. Got the Mag-hytec cover and love it!

Easy maintenance but they're pricey.
Nut

Is the OEM Motocraft a synthetic? I have read some good things about the Amsoil as well, just can't find it locally, only online.

PSNut - Is it true that when I put my Mag-Hytec diff cover on it will hold 6 quarts of fluid? That is more than some gas engines hold in oil!:9

Yes. Ford found that using syn totally cured all their rear diff failure problems.
Haven't tried it yet but i hear the Schaeffer 75W-140 gear lube is the best out there...check out this article: http://www.bobistheoilguy.com/gearoiltest.htm
Nut

no, it will hold ( i believe) an additional 6 quarts of oil. the stock pan, when drained, will net about 5-6qts.


our v-10 automatics that have the 4r100 will hold the same amount of oil that my truck's transmission will, and they only hold 6qts in the crankcase.

Sounds about right...hmmmmm...maybe 5 quarts. Can't remember offhand.
Nut