API doesn't have a motorcycle rating, only JASO does. As long as the oil isn't the energy-conserving kind, which will interfere with your clutch, you can use it. There can be some differences between oils for auto engines and those rated for motorcycles, mostly because auto oils don't have to tolerate transmission gear forces. Oils meant for Diesel engines do see those forces, which is why Rotella T6 now has a JASO rating for motorcycles. Whether those forces create significant changes over the time your oil is in the engine is up for debate.
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'68-7? CB450 and '98 Valkyrie Tourer, too