In 20W-40, not many choices are there i think. It's either Zic 4T or Total 4T and i have good experience with both of them in my GS.
There is atlas honda genuine oil too.
But now i am quitting Zic as it's gone API SL now, and these new grades come with less amount of some critical additives necessary for protection of old design engines like ours. I will be using Total most probably from now on.
My Advice: Use SL only if manual calls for it. Else best stick to SF/SG grades.
Anyway CG goes happy with 20W-50 too. It's the Suzuki bikes which give a little nakhra on thickish oils.
Here is piece from "Motor Oil" Article from wikipedia. A must read before you go putting SL in your ancient technology bikes.