Yannis, Moribundman,
Thanks for clarifications. Now I see the issue is right choice of HT/HS, and only then the shear stability related to the loss of kinematic viscosity.
Moribundman,
It's evident that your nice car is well maintained. Given your drive style and outside temperatures oil consumption is quite low. It means you used good oil and may expect relatively low wear (so not clear why you have decided to switch from 5W-50 to 0W-40). I would not deny a possibility that your usual high RPM helped you (to certain extend) to drive safely with 0W-40: if the wear is not big and oil pump works well, then at high RPM the shape and motion generates oil pressure and lubricating fluid film separates metal surfaces. This is so-called hydrodynamic lubrication. In addition, if the oil has low viscosity, but thick enough, it runs quicker and cools better. Lower oil temperature causes a consequent rise of oil viscosity. Probably it was your case with 0W-40 (that's why I said "to certain extend"). But once this will not work in such tough conditions even with reliable Mobil1 0W-40, moreover the car, though well maintained, is not like a virgin.
Now OEM requirements, think Audi specifies VW 502.00 or 501.01 oils for your car where HS/HS > 3.5 and following SAE grades:
5W-40, 10W-40 till + 15 C
5W-50, 10W-50 over + 40 C
10W-60 over + 40 C
15W-40,15W-50 over + 40 C
Usually synthetic oil provides better viscosity at high temperatures then the next higher mineral viscosity grade, so many users would be satisfied with 5W-40 synthetic. However, majority of latest 5W-40 formulations have HT/HS only 3.55-3.85 to pass Energy Conserving credentials. Thus there is no a big difference with 0W-40.
To be on safety side I would chose something with HT/HS about 4 or higher. My personal preferences in your case would be:
Total Quartz Racing 10W-50 HT/HS 4.5
Mobil1 15W-50 HT/HS 5.1
Castrol 5W-50 HT/HS 4.2 ?
Mobil1 5W-50 HT/HS 4.2
Castrol TWS 10W-60
Think that Redline 10W-40 would work too, but over their cite info I have not found something proving it outperform other oils. And don't forget to check first its compatibility with elastomers.
As for Pentosynth 5W-40, unfortunately I cannot make comments about it. If you decide to use 5W-40, check also Agip TechSint or HTS 5W-40. they should be available in USA.
I suppose if you consider all pro and con, you will stick with Mobil1 15W-50, known, reliable and available everywhere. Plus, you will have a chance to play with 15W-50 and 10W-30 grades, if you need something ligher for so called winter.
Yannis,
Sorry not to reply yet to your question about 300V. I was not fully satisfied, but for other reasons. Would we discuss and exchange with our info creating a special 300V topic ?
