Originally Posted by Glisse

Replacing the gear oil is certainly not on any Audi maintenance schedule. But plenty have done so, possibly this was preventative maintenance when the transmission was being highly stressed (very high output S6 modifications, way over the recommended torque ceiling, etc). Or perfectionists who argue (probably rightly) that there is no such thing as a lifetime fluid. I suspect changing the gear oil is a simpler process than the ATF, and there is no filter on the gear oil side. It is very rare to hear of gear set / drive shafts issues with the DL501, it is usually the Mechatronics side that has an issue. But I thought North American C7 A6/A7 with the 3.0TFSI were fitted with the ZF8HP - maybe this happened later in the model life?

Not sure if you are referring to the Sport differential, as they are the only ones I've used for the last decade or so. I have read anecdotally in forums that it is 100k miles, but I have never got close to that so don't know personally. I was never convinced the 100k miles figure was an actual Audi service requirement, though. As the centre diff uses the DL501's gear oil, and that is lifetime rated, I believe Audi also regard the rear differential oil as lifetime rated. Audi require only the ATF and lower filter in the front section of the DL501 to involve a service interval.

Originally Posted by Vodka G

thanks! So sounds like the rear diff (for the rear wheels) and the center diff (torsen) doesnt need to be serviced (unlike the haldex unit which i recall is every 20k miles) and it doesnt sound like it CAN be serviced by DIYer

Yes the ZF8 speed transmission fluid was also suppose to be "lifetime" and I am one of those that doesnt believe transmission fluid should be "lifetime" so 100k it is.