you have lots of options, if you just wanna make this very easy for yourself, you can go with GC or M1 0W-40...Redline recommends 5W/0W-40 grade oil for this engine, buddy of mine with this same engine uses, M1 0W-40 and Redline 0W-40, 50-50 combination...
I have currently switched from GC to 4.5L of M1 ESP 5W-30 + 2L Redline 0W-30, I do not have an M3 but 1998 BMW 328is Coupe w/M-Performance Package..
Please mix oils at your own risk...
back to the original point
GC 0W-30
M1 0W-40
Great Choices due to being widely available!