S8 torsen/differential fluid
 

Hey,

PLEASE can someone sum up what fluids I should use in TORSEN in the center and in the FRONT DIFFERENTIAL?

I read a lot of sites online and now I'm just confused.

any help appreciated 😁 thanks

For a fluid change, use standard synthetic 75w90 gear oil in all 3 differentials. Do not add, or use any friction modified fluid.

Anyone who tells you ANYTHING different is incorrect.

There is a documented tech note saying not to use the lubricant with friction modifier to refill the center differential. Unless there is major service.
. My car made a grumble when making tight U-Turns at low speed. Felt like a normal LSD with 2 wheel drive. No pops like many of my GM cars with LSD.
Had already flushed the front and rear with standard Audi lubricant. It was a battle but made a J shape nozzle out of thin copper tubing. Fished it down but was not tagging bottom. Decided to pump fresh fluid to circulate old. Some nasties came out.
Grumble was reduced. Mailbox was test track, make a U-Turn there if coming from home. Decided to modify the snorkel to try it again.
Second time got pipe to where I could feel it hitting the bottom. Rigged up the Miley Vac and slowly extracted the fluid. Clean oil this time.
Refilled with hand pump and been driving it for five years. Still grumbles when making a tight U-Turn. Just not as loud as it was originally.
I used the expensive stuff. I think MP told me the Center oil was different than the front and rear.
I am not a great mechanic. Took a while to find the Torsten. Had a drawing, but did not look like it from laying on my back with Rhino ramps. It is behind a support too.
I think the Torsten is filled with Lifetime lubricant. I think only one other person here has said they flushed the center differential on a 4.2 V-8. Dealer will not touch it.
Lifetime lubricants are ones that when they fail you get another car. Audi called the transmission fluid lifetime too. ZF named it LifeGuard.
https://cimg7.ibsrv.net/gimg/www.aud...0d833974b.jpeg

These are what I used
 

Like Dave posted most likely any full synthetic 75w-90 is fine. Especially on a 15 year old car.
But I used the Audi stuff. ECS Tuning has both in stock.
https://cimg4.ibsrv.net/gimg/www.aud...4c4441363.jpeg

I have had my transmission worked on 2x requiring fluid changes and both times the center diff needed a friction modifier or the car would bind when turning. The mechanic didn't know what the issue was but I saw this in ZF docs clearly that Audi applications did need the friction modifier.

See page 9-10 on both the attached ZF catalogues for reference to Audi specific applications.

WARNING:
The "STURACO FM 1992" additive is only needed after renewal (exchange) of the Torsen differential.
After assembly, pour the additive directly (axially) into the Torsen differential with the transfer case in a vertical
position.

Oh man, why does nobody get this???
 

As it very clearly states, friction modifier to be used ONLY after a renewal(exchange). So that means the first fill for a new/rebuilt Torsen diff. NOT for a routine fluid change.

Putting in friction modifier as part of a routine fluid change messes up the friction modifier ratio (as it stays behind “stuck” to the gear sets, case internals, etc), which then stops the Torsen diff working correctly. Sure, you’ll get less grumble whilst doing u-turns, but that’s because your center diff isn’t working right anymore, and is basically just acting like a standard open diff. Goodbye what makes quattro, quattro.

The binding was pretty bad without that friction modifier and the car is driving normally like a luxury car in both cases with it added, like it always did beforehand. There was transmission out work done in my case by a good ZF shop but no rebuild of the Torsen diff itself. I had leaking seals in this pipe that connects to the front and mid differentials (but no fluid inside that connected them or something like that). In the last iteration I drove it about 100km without the FM and I cringed every time it made that binding because it was just a nasty noise that made me imagine gears were grinding. It wasn't just a U-turn, it was any time I even made a moderate turn. When they added it the difference was immediate. Its been running flawlessly since, about 25,000km worth of driving in all conditions.

To each his own, just sharing my experience. Maybe in the OP's case he can start without the FM as you suggest and add it if it isn't working as expected and see if that makes a difference. Fluid can be drained/flushed so it seems a reasonable and safe approach if he runs into any issues.

Tranny out
 

Good to know. I think your experience is telling. Audi recommends the G055145A2 for the front and center on S8 and W12 D3 cars. They share fluid.
BTW, there is no pipe connecting the front and center differentials on 4.2 D3. The two are separate. And dropping the transmission to change fluid in the center might be how a dealer or ZF shop would change the fluid. Several solid mechanics( Mishar for one) tried and never were able to service these on 4.2 models.

Can’t speak to your specific situation, or what went on with your trans repairs, but wouldn’t be surprised if there’s more to the story than your mechanic let on.

Anyway, I changed my center diff fluid with just standard gear oil, as recommended. No issues, or any notable changes afterward.

https://cimg6.ibsrv.net/gimg/www.aud...b89e132cc0.png
That's where the pipe is and the leak was the one closer to the front diff. IIRC, there isn't any fluid traveling through that pipe.