best type of oil for 71 340 engine

i ran royal purple 10-40 xpr synthetic for 9 years on my 70 340 never a problem when torn down to add new alu heads and check **** out at 7,000 mile it looked new inside almost no wear on any of the parts . i added zzdp the first 3 oil changes nothing after that ,
let me add every engine is different no idea how yours was built , just stating my experience