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Oil weight doesn't have much of anything to do with idle. It only affects cold starts if you're well below freezing. Other than that the difference in absolute viscosity at room temperature of 10w40 and 15w40 of the same brand is probably less than the difference between 15w40's of two different brands. There is absolutely no way you can tell the difference between 10w40 and 15w40 when starting and idling except if you're starting in below 0 temperatures. And I know prodigit isn't since he's in Miami.
On my 50cc Chinese scooter I put in whatever oil I had on hand, it ranged from 5w20 since my Ford Focus runs on it, to 10w40, Rotella 15w40 when I bought it for my other motorcycles, and even 20w-50 just because my lawnmower runs on that stuff. I personally never cared since even if it did blow up a new engine costs less than an overhaul. And I've never had a starting or idling problem starting in temperatures as low as 40 degrees.
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