I have seen a BMW document issue (10/2004) dated 01.11.04 concerning the correct oils for use in BMW M3/M5 cars.
It states the following:
BMW Longlife 01 oils to be used in all M series engines except
M43/CNG
S54
S62/E39 (up to 02/2000)
Where Castrol 10w-60 is recommended.
BMW Longlife 01 Oils are of the following viscosities/specifications:
LL01 SAE 0w-30, 0w-40, 5w-30, 5w-40 ACEA A3/B3, API SJ/SL/CD ECII
I have a list of approved oils.
There is also a Technical Bulletin to Dealers dated 03/11/2004 concerning ALL M5's S62/E39
from 02/2000 onwards use 0w-30, 0w-40, 5w-30 or 5w-40 BMW Longlife Approved Oils
up to 02/2000 use Castrol 10w-60
This is stated very clearly and is the definative answer from BMW, your Dealer should be aware of these recommendations.
Cheers
Simon