I use Classic Car Oil in my classic cars. It is 15W40 and has the level of ZDDP and detergents recommended for classic car engines. It also has additives that are helpful for cars that are not driven daily. ZDDP levels are thought to be very important during breakin periods on a rebuilt classic car engine.
Most modern oils have their additives focused on not damaging the catalytic converter or getting better gas mileage.
Larry
Larry Durocher
1956 Mark II C5601429
1962 RR Silver Cloud II drophead
2006 Ford F150
2010 RR Phantom drophead
2013 Bentley GTC
2015 Lincoln MKT