Rear Diff Fluid Change Question

[ZcSpec]

I'll be changing my fluids this weekend, to include front/rear/TC/tranny. When changing the rear diff, the manual instructs to use ***EDIT*** 75w140 which I purchased (Amsoil brand). The question is, do I need to add any type of rear diff additive? I read somewhere and watched some DIY videos that also included additives for the limited slip, etc. The manual doesn't say to add anything so I am assume, use ONLY 75w140?

BTW, I own a Gen2 4.6L 4x4. Thanks in advance!

 

[swshawaii]

Regardless if the fluid, add one 4 ounce bottle of Motorcraft XL-3 friction modifier for best results with your non open differential.

 

[motormaggot]

are you sure the manual says mercon sp for the rear diff? i just looked it up and it says 75w140 synthetic. what manual are you looking at? might want to double check that.

front diff-80w90 oil
rear diff--75w140 synth
trans---mercon sp

hope you haven't done that service yet.

 

[swshawaii]

are you sure the manual says mercon sp for the rear diff? i just looked it up and it says 75w140 synthetic. what manual are you looking at? might want to double check that.

WOW! Differential saving catch. I'd give it only minutes using Mercon SP. Buy this good man lunch!

 

[ZcSpec]

are you sure the manual says mercon sp for the rear diff? i just looked it up and it says 75w140 synthetic. what manual are you looking at? might want to double check that.

You're right Motor. Thanks for catching that!

I was referencing the ATF instead of the Rear Diff. To confirm, should I add anything to the 75w140 gear oil?

 

[motormaggot]

yes- install the friction modifier to the rear diff
glad we caught that.

 

[ZcSpec]

yes- install the friction modifier to the rear diff
glad we caught that.

Thanks a ton guys...I'll purchase a bottle before I do the fluid changes tomorrow.

 

[motormaggot]

 

[Anime]

It usually goes without saying that limited slip differentials require friction modifier, but these days, lots of gear oils say they already include it, but it doesn't say WHAT they include or what specs it meets.

You probably are best off just adding the Motorcraft XL-3 to the gear oil in any case.

You can use friction modifier in an open diff without issue, but using gear oil without it in a limited slip diff is sure to lead to very expensive and time-consuming problems.

 

[e_toften]

Just out of curiosity, why is everyone saying add the friction modifier to the open diff? It's not required unless you have a limited slip. A quality gear oil will handle anything you throw at an open diff.

 

[swshawaii]

Who's saying that? Nobody on this thread.

 

[e_toften]

ZcSpec never said he had a limited slip. My manual says the same thing as his. Mine is an open diff. The axle code inside the door will have an L in the axle code if it is a limited slip.

 

[swshawaii]

Got it, thanks for pointing that out. I'm naive enough to believe ALL 4x4 vehicles are LS. Not so.

 

[e_toften]

That's correct. Not all 4wds have limited slip diffs. Most Ford trucks have an L in the axle code on the door for limited slip. Fool proof way to tell is to look at the tag on the diff. Limited slip will always have an LS by the gear ratio.