Originally Posted by GregW / Oregon
Castrol TWS is far better than your garden-variety Castrol, as it ought to be for $9/liter. I would stick with this, as it is what you will get at the dealer for your free services, and the factory has extensively tested it. No sense to differ for in-between oil changes (to save $30?).
The E39 M5 crowd has flogged the engine oil issue to death - the BMW TIS allows both 5w-30 and 10w-60 (!) and those who've tried both didn't find much of a difference in driving experience or in UOA.
TWS is what BMW Motorsport (not "M" but the actual racing division) specs for most of their engines, except for the Formula 1 V8. It's installed and sealed inside Formula BMW engines, it's spec'd for the E46 M3 GTR V8 and it's in pretty much everything else they sell so race cars can race. It's on par with the best racing oils in the world. That it's as cheap as it is is surprising.
I went through the gamut of different (allowed) oils on my E39 M5 - nothing did better than TWS although I did find a few that performed just as well. So why mess with oil selection? There's nothing better and it's free if you follow the maintenance schedule!