Below is a link to a page talking about the Kymco scooter manual stating that a SG-rated 15W-40 oil must be used. Turns out, the SG rating for oil is OLD (I think it's now up to SM rating), and the Kymco rep states that a regular 10W-40 automobile oil should be just fine.
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1692715Now that we've established that we CAN use either 10W-40 or 15W-40 oil in our scooter engines, which is the best one? Well, that's a tough thing to figure out.
As for me, because it's a high-revving engine, I want to be sure that there's no foaming, and little viscosity breakdown over the life of the oil.
So, after engine break-in (in which I'll be using a conventional 15W-40 oil and changing the oil and filter very frequently (ie.: after ~25 miles, then ~50, then ~100, then ~200, then ~400, then ~800, then ~1600), I'll be using a diesel-rated synthetic 10W-40. Oil and filter change interval after break-in will be every 1000 miles.
The diesel-rated synthetic oils not only have more detergent in them to keep the engine clean, but they've got anti-foaming agents, and stand up to extreme use better, so there's less viscosity breakdown.
Keep in mind that motorcycle oils don't have the friction modifiers that automotive and diesel oils have, because those friction modifiers can cause the motorcycle clutch to slip. So they're less 'slippery' than other oils. If you have metal-to-metal contact (piston scuffing, etc.), you won't have as much protection as with an automotive or diesel oil.