my oil has been breaking down rather quickly...
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.