unless it is real cold 10-40 should do fine. i run yamalube in all of mine and it seams to hold up to the massive amounts of heat these machines create when you run them hard and should be able to start and oil correctly down to at least 20 degrees probably more. but as long as it is oil designed for wet clutches you will be just fine. i am sure to take a beating for this but i think unless you are racing and trying to squeeze every last ounce out a motor synthetics are a waste of money.
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