Yamalube 20w-40 was the Yamaha standby for years. I think somebody once advised that it was a Citgo product. Not sure how accurate that was... Anyway, no reason not to use a heavy-duty engine oil such as any name-brand 15w-40 or 5w-40. Some people claim that the synthetic version shears out of grade too fast in a shared-sump, wet-clutch setup such you & I have. My bike prefers either Rotella T Synthetic "T6" 5w-40 or the semi-synthetic "T5" 10w-30. I think it runs better on the T5 actually! Anyway, I ran the conventional 15w-40 version & the shifting was a bit degraded. No big deal - just changed it out a bit sooner. It was still cheap!
Also, as mentioned, Mobil 1 15w-50 (used to call that one 'Red Cap' once upon a time) is a heckuva bike oil as well. I hear good things about Valvoline's line of conventional bike oils. I used Castrol 0w-30 ("German Castrol" around this forum) & it was terrific.
Try 'em out. Let the clutch action tell you when it's time for a change. You will be way ahead, economically speaking, than if you went w/ a bike-specific product in your application.
John.