http://www.widman.biz/uploads/Corvair_oil.pdf
Conincidently, i started reading this later after i posted, read it all, about oil , noise, etc why not to flush etc, purely fantastic article.
If you too lazy to read it all, read Page 5 and page 17 points 11 & 12. Will answer it all for you.
P.S on viscosity, 5W30 are the norm due to lower temps and extreme variations throughout a day. 10W40 and 15W40 would be our equivalent of 20W50/25W50. Australia has higher ambient temps than U.S and we keep our cars for longer.