I ain't read ANY of the replies. C'mon man, four pages??
But to answer the O.P., it's usually best to follow the ENGINE mfg. owners manual. They all have a chart that tells you what to use. When in doubt, use 10w-30. In the warmer months I always use/recommend straight 30 wt HD.
" But let me tell you something ,folks: You can't fix stupid!
There's not a pill you can take, there's not a class you can go to. Stupid is forever!"