I’ve been running Castrol GTX 10w-30 in my ‘73 Dart slant six since 1991. At that time I rebuilt the engine (169k mi) and found a lot of sludge (previously used Havoline). Did a partial rebuild five years ago, zero sludge and minimal wear. Currently at 387k mi. So I’m satisfied. I have some of the Castrol Classic with ZDDP and will be using it soon.