The clearances back then were more than now. Rings were different and the seals also. Multi weight is much better oil now. I still have never had any luck with pure synthetic. I built one brand new engine, new pistons, ground crank, new rockers, new cam, new lifters, new push rods. After running regular oil for break in of rings. I switched to synthetic Mobil 1 took a trip across the U.S. several thousand miles. By the time I got home the valves did not sound right so I pulled the cover and the push rods were almost worn to a point. I had to do everything all over again used the same brand of parts but put regular 5 W 30 in and went over 100,000 that time.