First of all, the other brands of 10w60 mentioned would certainly work fine in the M3, but are they better than the TWS? Where's the data? A single pass uoa with a different oil means nothing. How about a voa (virgin oil analysis) of the "better" oils. In what respect exactly are they better? For comparison here is a recent voa of the currrent formulation of TWS http://www.m3post.com/forums/showthread.php?t=839626
You can certainly run a lighter grade oil in the M3 (e.g. Mobil1 0w40). As a matter of fact a recent uoa of the factory fill drained at 1200 mi seems to show it is a xw30 wt oil, and a fairly unremarkable one at that (it may even be a conventional mineral oil, as many break-in oils are) -
FWIW, I think the reason BMW specs a 10w60 for the M3 is to counter the thinning effects of fuel dilution over the span of an extended drain. Fuel dilution is not that uncommon in high perf. motors like the s65. In fact if you look at the pds (product description sheets) of many "racing" oils (e.g. the Motul 300V series) you'll see they are intended to deal with fuel dilution. Many uoa's of TWS have shown that by the time the oil has run around 7k mi it is a heavy 40 wt. I don't recall ever seeing a uoa of an oil like Mobil 1 0w40 run to 7k or more; wonder what the viscosity would be at that point?
Franky, I think you could probably run Wolf's Head 5w30 in the M3 and get away with it as long as you kept to very short drain intervals.
As for the indie shops that say their brands of 10w60 are better - - ask them to show you the data. Tischer BMW (aka Get BMW Parts online) have said that the reason they don't offer TWS on sale at lower prices is because their profit margin on TWS oil is so low to begin with. I suspect that is the reason many indies don't offer it but sell other brands with a better profit margin. Still good oils, but better than TWS?
PS for W ///: if you want an oil to run at higher temps, you want a thicker oil like TWS. Thicker oil has more internal friction than a lighter oil and thus runs slightly hotter.