Bronze

I discussed this with some guys at work, and we just don't see why there would be magnesium in a brake system. Magnesium is very light and strong, but expensive. So far, in automotive applications its usage has been limited to wheels, structural cross member parts, and cast engine parts. It seems unlikely that Audi would put an expensive metal like magnesium in their brake system.

I did some searching, but didn't find anything new. I did find that there were some issues with magnesium-containing road solvents that created brake issues back in the late 90's, but nothing related to magnesium in the brake system and fluid compatibility.

A4NCAR

Still waiting to hear back from my VW/Audi inside source with whether or not it is even in there.

Bronze

need to change the brake fluid until my first track event in August, so I've got time. Worse case, I can use 5.1 and give the 600 to someone around here that uses it.

I'm ordering some of your highly recommended Ester E-tech 0W-40 for my 30k in a couple of months. :-)

A4NCAR

The DOT5.1 can be pushed a bit harder then OEM spec'd BF, so it's enough improvement for my street driving. I'm sure some limited track use would work too. But the RBF600 would be more suitable.

I think you'll be happy with the Ester E-tech. I have been so far with it in my 1.8t VW. I used the 300 V in my A4 and it seems to be an amazingly smooth oil. I noticed a slight improvement with engine idle (reduced vibration/resonance) in both cars. Just seemed a little smoother running overall. Cold start up is smoother/quieter too (it's that ester coating all the surfaces and not flowing off after sitting).