Best oil for initial start up, high zinc based mineral was planning on a short initial run and tune up then drop oil, refill and 500 mile then change again. Was lucky in some ways mate was housebound so dropped engine round and he replaced bottom end. I only needed to fit new rings and clean everything up. Replaced seals and back together all started with engine clean, fitting new clutch and got carried away...
Valvoline VR1 20w50. High zinc quality oil. Did you glaze bust the cylinders with changing the rings? To bed them in you need to do loads & overruns to get some pressure behind the rings to bed them into the cylinders.
I had it written in black and white on the 12 month warranty of my brand new JK Preservation Parts engine to use 20W/50 oil or void the warranty. For most people it is easier and cheaper to get the Halfords Classic oil than try and find a modern oil with the flat tappet zinc ZDDP additive at the correct percentage, I expect that is one reason they called for 20W/50.. I was also limited to 50 mph running in which made going to Techenders a bit slower one year. Plenty of high load varying rpm was managed.
