Warranty on my car is out so I was wondering is it harmful to use 5w-50 castrol synthetic oil instead of the 10w-60 that bmw uses?

Even Ferrari only uses 5w-60(shell) synthetic in the F430's. Do I really need to pay $13 more per liter(cdn before tax) for 10w-60? I don't track or autocross(yet) my car. Just DD with highway sprints.

Oops I should mention, my car is a 2001-2006 style BMW M3.

Thats your choice to make.
But, from what I vaguely remember, that 60wt shears pretty quickly into a 50wt anyway.

So, if you plan on a somewhat sane interval, I would consider the 5w50 from Castrol, Maximausa, Torcousa, or Schaeffers.

Another great advice by Undummy...

@jotti: nice car, nice engine, good technique, but still I would recommend the 10W-60 for you. It's simply the best for your highrev-engine. It's not for nothing BMW only recommends a 10W-60. It's better to pay a few bucks more than wrecking your lovely M3 engine.

If you want I can send you the complete OEM report on this model.

A full ester 10W60 synthetic still shears when used in MGB racing but I would stay with the recommended visc.

Use Mobil 1 15W50 Extended Performance instead, and have an oil analysis done to verify the oil performance. At about $27 per 5 qt jug at Walmart, its a less expensive alternative.

However, it'd be wise to stick with the OEM 10W60 until you're out-of-warranty, as some of the M3s were recalled to have some engine components replaced (due to some defect) and the warranty was extended.