As mentioned by Chip, the correct oil is the TWS. It was designed by Castrol in conjunction with the M3 and M5 engines.

BMW themselves for a while said that the later M5 could be used with the newer oils that they used in the other cars, but then realised that actually, the TWS was still the superior oil for the car.

Pissing expensive if bought from BMW (about 15 quid a litre!!!!), and this is the only over the counter place to buy it from.

I buy mine direct from Castrol Classic Oils. They are found online, and are easy to deal with. The other alternative is Opie oils.

Although many say that Castrol Edge 10W60 is the same (which it might well be!), I still use the original TWS that was specced for the engine. You cant go wrong then, can you!

As an aside, the TWS was imaginatively named (Ten Weight Sixty)

JJ