Rear Diff Oil

**0396** · 10-26-2008 05:39 AM

Sorry if this has been posted before, Has any one replaced the rear diff's oil?
If so, what grade of oil... I could not find any info in owners manual.

Thank you

**Daft** · 10-26-2008 05:52 AM

The rear diff can use GL-4 & GL-5 rated gear oils.

If you want to change it, Redline 75w90 synthetic is a good option.

It carries a GL-5+ API rating and is fully synthetic.

BG UltraGuard is another good option in the GL-5/Synthetic categeory.

**Note: Do not use these oils in your manual transmission. They are too high in sulfur content and will soften the brass/copper synchros.**

If you need a transmission gear oil, BG Synchroshift II or Redline MTL are the right ones to use. Redline MT-90 is an option too but, it is slightly thicker and will cause notchy shifting in cold climates.

**0396** · 10-26-2008 05:59 AM

Originally Posted by Daft

The rear diff can use GL-4 & GL-5 rated gear oils.

If you want to change it, Redline 75w90 synthetic is a good option.

It carries a GL-5+ API rating and is fully synthetic.

BG UltraGuard is another good option in the GL-5/Synthetic categeory.

**Note: Do not use these oils in your manual transmission. They are too high in sulfur content and will soften the brass/copper synchros.**

If you need a transmission gear oil, BG Synchroshift II or Redline MTL are the right ones to use. Redline MT-90 is an option too but, it is slightly thicker and will cause notchy shifting in cold climates.

Thanks for responding... I have Mobil 1 and Swepco 75-90... what's your thoughts on these? New to working on my 04 S/4 with 34k miles..

**Daft** · 10-26-2008 06:35 AM

Well, 34K miles is a bit early to worry about gear oil. I recall the replacement interval being almost double your current mileage.

The Mobil 1 is fine as it is a synthetic GL-5. The Swepco stuff appears to be commercial grade gear lube that is made for heavy trucks. I wouldn't use it.

Do not use either of those gear oils in your transmission.

**0396** · 10-26-2008 09:02 AM

Thanks for the education... Guess I will wait unitl 64k before I change it- :)

**tatarin** · 10-26-2008 05:54 PM

NO...

don't use redline. only if you want a new diff in like a year after running their fluids.

Amsoil 75w-90 that's it, yes it's more money but it's worth every penny

**P3RryaudiA4** · 10-26-2008 06:00 PM

i agree redline made my tran even more notchy, i flushed it out and used royal purple 75 90

does any1 know tho, is it better to let all the fluid drip out of the fill hole till there is nothing coming out anymore, or to just put the plug in when it starts to come out?

**tatarin** · 10-26-2008 06:11 PM

^ just when it starts to come out put the bolt in

**FabulosoM3** · 10-26-2008 09:59 PM

i used motul gear 300 75w90 and its been fine i replaced it at 75k along with my tranny oil which uses the same fluid.

**c4racer** · 10-27-2008 04:24 PM

Originally Posted by FabulosoM3

i used motul gear 300 75w90 and its been fine i replaced it at 75k along with my tranny oil which uses the same fluid.

The manual trans also uses GL-5 synthetic oil? I thought it used something different from the rear diff?

I need to do this - close to 65K now and been meaning to do it since 60K but haven't got around to it.

Any reason to use factory fluids for trans or diff? Other than funding the dealers parts dept to keep them in business?