Amsoil 5/50 Signature Series (AMR) since last spring. 5/40 Euro Classic (EFM) last 5 years prior. Partly because it's not the low SAPS 505.01 OE spec. Even if the oil was free I simply won't run a stripped down OE spec oil in a 15year old tuned engine. Of course not all OE oils are equally poor. But they are simply regulated to PPM levels. They can do all they want with the base stock oil but that interferes with the pricepoint in the big box retailers. If it meets spec, they'll ship it. Legally unencumbered as well.
Interestingly enough Amsoil is the only company I've seen the has a written warranty policy. It's actually fair, and takes responsibility.

At this point we're trying to protect the engine. Not the emission parts. OE oil is specifically designed for the cat. converters for 8yrs/80K miles (10/100K?). What is the engine warranty? About half of this - how interesting.

Seen a lot of oil come out more than a few RS6's, including some of the brands posted above.

Only other oil I would mostly trust is Redline. Have used most of the common Eurotrash lubes like LiquiMoly, Motul, Pentosin. Results vary- nothing bad but not what I'll run for 5K miles. All the Euro OE spec oils are stripped down. A diesel spec oil is a good substitute, at least in terms of additives - BUT try to get an older spec. It will have better HTHS to keep those cylinder walls nice and smooth. I need to study up on the latest 5/40 diesel oils. Have a VW V10 diesel to care for now. Poor thing had 5/30 OE in it. Bit thirsty at 14.5qts service fill. (17-19qts total?). BTW a RS6 takes almost 11 qts if dry. That's 2-3 qts of used oil getting mixed in every time. Change it often to keep the average contamination down. UOA is cheap now.

After not getting bitten last 20yrs and the hundreds of other cars I've changed to Amsoil it's an easy choice. More than a few cars I certainly can't afford. After seeing what comes out one thinks that anything would be better...

The 5w40 Euro Classic is a great oil. Very affordable, even at full retail. The fancy 5w50 is definitely better, far beyond OE spec oil. If you're tracking the car or running hot in the south, it's the oil. (Originally designed for the Nissan GTR)

As most know I purvey this fine snake oil (see sig). Some nice discounts are available.

Mr. Snap-On truck just pulled up so gotta run. Cheers everyone.