There is a Havoline Motor Oil which is 20W-50 and it's widely available.
But i am not using that one. GS behaves better with 20W-40 for me.
There is another Havoline oil which is proper 4T motorcycle oil and comes in 20W-40 viscosity.
Yes i used Zic M5 for long time, but it became harder and harder to find here and then blew my rings/valves too so i lost interest in that oil.
20W-50 is supposed to be thicker when in hot condition. Actual viscosity varies brand by brand.