Ran a 64 Spit with 20-50, had an 80 Spit and ran 20-50. The 80 has hardened valves, don't need lead. My wife ran a 73 GT6 for 10 yrs, I used 20-50 without hardened valves. Ran the begeezes out of it, never had problems with engine. Just u-joints and blew one tranny.