I've been using 20W-50 in my '56 Cadillac for over 20 years and this car takes regular trips to Florida and back at 80mph with no adverse effects.

I use the heavier oil because of its the sticking power on the wear surfaces and because it fills up the increased clearances in old engines. It also slows or even stops some oil leaks.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)