Complicated subject, and you'll probably get a bunch of answers. None of the problems with oils whose rating starts with "S" are wet-clutch related, at least not in the viscosity grades commonly used in motorcycles. Those problems come from oils that are "ECII" (energy conserving 2) rated, which contain friction modifiers, and are generally limited to oils whose upper viscosity rating is no more than 30 (ie 0w30, 0w20, 5w30, etc). The heavier weights (like 5w40, 15w50, etc) generally aren't used in gasoline passenger vehicles. Well, that's what I understand about it anyway. Someone will probably be along to clarify this.
Now, "S" vs "C" rated oils (ie 10w40 SJ vs 10w40 C*). I'm not on expert on this, but as I understand it, until recent changes oils for diesel engines were still allowed to have higher concentrations of extreme pressure (EP) additives, like ZPDP, while the later car oils rated, say, SJ, SK, or SM, were not allowed to have so much (or any?). That is why you can still buy (mostly motorcycle-specific) oils that are rated SG, for example, because they contains these additives and thus don't meet the requirements for the newer ratings. As I understand it the latest diesel oil rating (S?) don't have as much or any of them either. I'm sure someone will be along who knows more about this.
Long story short, to be safe you can use one of the motorcycle-specific oils that have the earlier rating. These are somewhat more expensive, although you can get conventional motorcycle oils for less than $4/liter (which is still about twice what you can get decent conventional car oils for). I've used a lot of Castrol Grand Prix 10w40 bike oil and found it acceptable. The oils sold by Yamaha, Suzuki, Honda or Kawasaki are all fine, as well as any of the other bike-specific oils. Actually, given all the stuff I discussed above, it may only now be the case that you can really justify motorcycle-specific oils, as newer-rated non-MC oils like Mobil-1, Delvac, Rotella, etc, may (and I stress MAY) no longer have all the stuff one would like to have in a bike oil.
Of course, if you're not too hard on the engine, it's quite possible that you would get quite acceptable performance out of any oil of the suitable grade.
