For the Castrol trivia buffs, TWS 10w-60 IS the "old" Formula RS 10w-60, from 1999 and before.
Apparently when Castrol reformulated RS, BMW decided to keep the old formula alive for their Motorsport division. That's why TWS's full name is "TWS Motorsport", and why it's only available from your friendly BMW dealer.
There's a BMW TSB dated October 2000 that says that "M" engines can use either Castrol TWS or RS, provided that the RS is obtained through the BMW logistics chain and has a BMW part number. I had a bottle of Castrol RS that I got from my dealer, but the next time I went in, they only had TWS. The Service Bulletin says they're identical except for the label - even the BMW part number is the same.
Anyway, TWS is a 10w-60 with a pour point of -57 degrees C, so whatever the base oil is, it's pretty darned good. It looks like a thicker version of SLX 5w-30, our trusty "GC".
Cheers
JJ