OK, how many guys know of any motorcycle, Vee, Wee or other that has blown up, ever, because of the "wrong" oil?
If yes, I'm sorry to hear it, if no, I rest my case.
Even running the "car oils" that are non-energy conserving and supposedly don't have all the right additives for our bikes valve trains (cam lobes esp.) has never caused an engine to wear out that I am aware of. Maybe extending it out well past 100,000 miles might add a few examples of failure, but oil is so good these days that we are doing more for our own minds than we ever are doing for our bikes.
Hey, I am one of those guys that wants to put the best into my pride and joy, but reality says the bike can't tell the difference from a longevity standpoint. I totally agree that different oils can help/hurt shift quality, etc but I'm talking engine longevity.
BUT, for the sake of the intent of this thread, I use Rotella 5w-40 in the colder months and back to a 10w/15w-40/50 in the summer, all depending on my mood at the time of the oil change. Currently, I am running a NAPA 15w-50 full syn. that I picked up last year for the heck of it. Ran it for about 4,000 miles last summer, then ran the above Rotella for the next 4,000 miles or so, then back, to the last 3 qts. of the NAPA oil this summer. FYI, when I put shims in during the valve adustment, the cam lobes looked great and the bike is just about to turn 27,000 miles.
Just goes to show you how much the oil companies put into their ad campaign to make us think it is such a big deal!