[Scharbag]

Red Line 5W50 is very close in spec to the Castrol TWS 10W60 originally used by BMW. Red Line 15W50 and 5W50 are close to the same oil based on the specs from Red Line's site.

As for flow, the S65 has a variable volume oil pump that maintains a pressure set-point. Oil pumps do not "make" pressure. Oil pumps produce flow. Based on the system dynamics and the oil viscosity, pressure will result. Lower viscosity oil MUST have a higher flow to maintain the same system pressure. The S65 pump is a pretty good one and has been shown to produce good flow at high RPM as long as the bearing clearances are appropriate.

As pbonsalb said, the question is if a thinner oil can withstand shearing at high loading in order to protect from metal-to-metal contact. I am by no means an oil expert, but if I am reading things right, HTHS is a measure of the oils ability to flow while not shearing under load. Higher the number is better.

HTHS numbers:

Castrol TWS 10W60 - 5.2
Castrol Syntec 5W50 - 3.7
Redline 5W50 - 5.0
Redline 15W50 - 5.8
Mobil 1 0W40 - 3.6 (lots of people seem to happily run this oil in the S65)
LM 10W60 - 3.5 (wow - this is low for a 10W60)

So, while the 15W50 does have the highest HTHS, it does not necessarily mean it is a better oil for the S65. If you are really going to be tracking your car a lot, it is likely best to run a proper 50WT or 60WT race oil with very short OCIs as they are not designed for road use.

For daily use, and the odd track day, any quality oil that provides adequate protection against shear at high RPM will be fine.

I have happily run:

Red Line 10W60
Red Line 5W50
Castrol Syntec 5W50
Castrol TWS 10W60
BMW Twin Power 10W60
Liqui Moly 10W60

As long as I can access FCP Euro, I will run Red Line 5W50. If that goes away, then I will likely go back to running Castrol Syntec 5W50.

Cheers,