98 Ford Mustang gt rear diff

[MetalSlug]

I got the rear diff rebuild 3000 miles ago . The builder running some OREILLY 80w90 . I want to flush it and change to synthetic . I think the factory call for 75w90 . But dint ford switch over to 75w140 on all their 8.8 diff ? Thanks

My fluid choice either Mobil 1 or valvoline . Just need to know which weight.

 

[CharBaby]

Do you have an owner's manual?
Was the Diff rebuilt to factory spec's or was it BUILT?
Are you going to TRACK the car(1/4 mi or OVAL Track)?
Does the Diff have LSD?

Nothing wrong with 75w90 or 80w90 for the street. Mobil 1 is my choice however, others will chime in with more info.

 

[MetalSlug]

Originally Posted By: Char Baby
Do you have an owner's manual?
Was the Diff rebuilt to factory spec's or was it BUILT?
Are you going to TRACK the car(1/4 mi or OVAL Track)?
Does the Diff have LSD?

Nothing wrong with 75w90 or 80w90 for the street. Mobil 1 is my choice however, others will chime in with more info.


My owners manual say 80w90 . It was rebuild because it’s start whining bad . I think the shop rebuild closer factory spec . So 75w90 Mobil 1 then . It’s just daily drive

 

[CharBaby]

IMHO, YES!

 

[racin4ds]

Either will work fine. Ford did back spec many 8.8's to use the 75W140 syn fluids but I've never had an issue with 80W90 in any of my previous stangs.

Don't forget the friction modifier additive if you have the factory clutch pack locker (tracloc)

 

[CharBaby]

The Mobil 1 75W90 has the LSD additive in it already.

Just an FYI,
Funny thing I've read, many folks don't seem like synthetic gear lubes for one reason or another. There are some dino gear lubes(in which I use) that also have the LSD add in them such as PEAK 80W90 LSD from Pep Boys for about $15-$16 gal rather than $11-$12 qt for the M1...just saying as, IDK if a rebuilt Diff needs to be broken-in but if so, dino gear lube may be an alternative for several drain & fill for this process before using a synthetic.

 

[zeng]

Originally Posted By: MetalSlug
My owners manual say 80w90 . It was rebuild because it’s start whining bad . I think the shop rebuild closer factory spec . So 75w90 Mobil 1 then . It’s just daily drive

Checking with you ,did you use 75W90 regularly prior to rebuild ?

 

[Trav]

Ford seems to like the 75w140 in their larger diffs it works well even in cold parts of the country. I use Mobil 1 with Ford stink additive.

 

[SubieRubyRoo]

Gear 300 from Motul

 

[SevenBizzos]

I put M1 75W-90 in mine. Runs as quiet as can be.

 

[MetalSlug]

pick up 2 quart of M1 75w90 at auto zone, and a bottle of tranx. I guess this stay in there for good.thank you guys,