In 1973 I bought a new Toledo which indecently was gutless when compared with a new 13/60 I had in the 1967. However, the Toledo engine did 125,000 miles before I pulled it out in 1992. All the bottom bearings had worn out. However, the bores and top end were still in good condition. At that stage I changed every thing to 1500 with 85 BHP available. But that's another story. The point being I ran the engine on 15/40 Superlube. The interesting thing is the oil pressure would start to reduced when the mileage got over 3,000 miles and that's when I used to change the oil.
The point being there is nothing wrong with using the original mineral oil as long as you change it regularly. 125,000 miles on a 1296 large crank engine was OK my me.
Dave