where do you live? What do you temps get down to?
Refer to the owners manule I dont have it on hand right now to give you the temp/oil range but for me i use 10w30 yamalube from late summer to eailer summer and then i use 20/30 on the high 90`s 100degree days.
I live in georgia. I forgot how low temp can be for the 10w30 but i know in ga it should`nt get cold anof to use any lighter. I get back to you soon but tell us where you live.