What oil viscosity should I get?

I just got a 1997 BMW M3 with 178XXX miles and somebody I know told me I needed to change the oil with 15W-50 Mobil 1 synthetic since it's got so many miles. Do you think that's good because it seems a bit thick to me, but I don't really know...What oil viscosity should I get?
15-50 is probably thicker than you need. if it says to use say 5-30 i would only go to like 10-40 unless its really worn out and uses oil fast. the bearings will get looser with miles no matter what oil you use. 15w-50 is probably ok but i would only use it if it says to use higher viscosity or if you live somewhere hot. ive heard people say not to switch to synthetic after lots of miles but ive yet to witness it hurting.What oil viscosity should I get?
15w-50 is a bit thick, no matter the climate and will slighty decrease gas mileage. I don't think a synthetic will do much good with so many miles either but, using a quality oil is a must. Castrol or Mobil 10W-30 would be my suggestion. Both offer ';special'; high mileage oil brands.
It depends on where you live. In warmer climates that's fine. Did the previous owner use synthetic as well? If that is not the case you may want to be careful because synthetic oils have a high detergent content can you car can develop hidden leaks.
I would ask the shop you take it to when you go to have your oil changed. If they recommend it then it's probably not a bad idea.
Not a good idea. Too thick oil will kill the engine with lack of lubrication. I'd stick with 5W30. (I assume that's what the owner's manual says.) High mileage does not necessarily equal high wear on the bearings, etc.

blemishes