Synthetic oil use on flat tappet cam after break in

My dad still has his 1978 power wagon he bought new, 360.
First oil change back in 78 he switched to mobil 1 synthetic.
At 213,000 miles, it was leaking oil bad out the rear main seal, and soaked the clutch.
I pulled the engine, dad said re-build it. There was no wear on the bearings at all, cylinders still had cross hatch marks on them.
I just re-gasked the engine and put a new clutch in it.
It is still going strong with 240,000 on it now.