bmwm750

So, since I'm going to be doing my ATF next week, I figure while I'm there, it wouldn't be the worst idea to do the diffs as well. From what I can find, Audi says the center and rear diffs need their whatever fluid with FM modifier, while the front uses something else. Red Line says you can just use their regular 75W90 fluid in all 3.

Thoughts?

Brozee

So there is three total? I'm guessing one front diff, the distributor gear, and a rear diff? Is it easy to flush?

BTW I found this, shows all three use the same fluid. Mind you its for a D2 A8. Only difference I see is that the middle diff, aka distributor gear uses a different oil.
http://www.audipages.com/Tech_Articl...eckingrev1.pdf

Cheapest audi OEM fluid I could find is $26.
http://www.audiusaparts.com/parts/20...icleid=1431456

And cheapest middle fluid I could find is $70
http://www.audiusaparts.com/parts/20...icleid=1431456

mishar

Front and rear differential: G 052 145 S2

Mid differential: G 055 145 A2

You can use the first one all around, or any other transmission oil of the grade, but that would diminish Quattro function.

Only thing that puzzles me is that I remember reading somewhere that front and mid differential share their oil. I have to look into that.

MP4.2+6.0

Suggestion: always remove the fill port plug BEFORE you remove the drain plug. If you can't remove fill port, stop there obviously. On my C5 4.2 (different tranny), you cannot get to one of the front fill ports w/ normal parts (like exhaust) in place and tranny in car. Thus, I never touched it. Rears no issue, but look carefully in front.

Brozee

Here's Royal Purple's version of what we could use in the front and rear diff's. Not sure how much is needed to refill the different diff's in this car but this is a good alternative for the front and rear.
http://www.amazon.com/Royal-Purple-0...7635587&sr=8-3

bmwm750

I had planned on just using "regular" Red Line 75W90 in the rear diff for sure, and likely the front, but it sounds like perhaps it wouldn't be the worst idea to get some G 055 145 A2 fluid to use in the center diff. The fact that the Audi sheet which I referenced suggests FM booster for both the center and rear diff, should I use Red Line's 75W90 or the 75W90NS? It would seem that since the 75W90 has friction modifiers already in it, that it would be fine for all 3 diffs?

And yes, I learned a LONG time ago to ALWAYS undo the filler plug before the drain plug on these things (ask me how I know)!!!

bmwm750

Why do you mention "diminish the Quattro function?" Is the fluid used in the center diff THAT special in this case?

Brozee

BMW please let me know what you end up going with. This seems like something I can do in my buddies garage using his lift. Finally something I can do myself lol! Only thing is, if you need the special middle diff fluid, that is very pricey compared to the normal redline or royal purple. Anyone know how much is required to refill the front, the middle, and the rear?

Anyways, seems like either the royal purple or the redline would work out just fine. As for taking off the filler cap, taking that off first also helps drain the fluid out the bottom port as well. So good looking out!

bmwm750

I'll have this all done by the end of next week, and will report what I did and how I did it. Personally, I won't touch Royal Purple anything, so it'll be Red Line for me.

As to the "super exotic center diff fluid," you only need one quart of it, so while $70/ quart is ridiculous, the extra $50 over a quart of Red Line, on something you're going to do once over the lifetime of a car, is a rather moot point in my book.

mishar

Torsen differential limited slip is based on the increased friction in that particular type of gears. Audi special oil increases that type of friction while still lubricating bearings and other gears. Regular oil won't hurt it. It will be just less limited slip. Or less Quattro. And we payed for all Quattro we can get.