Why 20w50?? There is no reason to run that. Use a quality (by that I mean an oil with a base stock of Group IV and V base oils and not Group III) 5w30 and don't look back.
Unless you are burning methanol or something there is no reason to run that grade of oil.
don't thinner grade oils need tighter clearances like more modern motors have? they are designed for thinner oils at the factory so an 'old' motor would need to be built with thin oil clearances in mind.
neil.