Something I just caught a little while ago is that the Royal Purple Duralec 15W-40 is *not* API rated for gasoline engines -- like API SN or similar. How much of a problem is it really to run a diesel-only rated oil in an air-cooled, gasoline engine? Obviously, universal diesel oils are OK as they're designed to run in both engines. But what harm could really happen from one without an API SN or similar rating? It won't be as clean, I assume. And that's about it??