Some belts have "ribs" on the outside, and then ridges on the inside.. I would tend to stay with the OEM stuff too, it's not that expensive, it's just a belt.
'86 325es (TURTL3): PJ peetsa hawler
'87 535is (BADWATER): almost done... really.
'86 325e (GRN HLL): DD/show/rocket
'04 x3 3.0i: Wife's Bimmer (manual, woot!) My BMW Blog