Hi Npw
At the recommendation of Ken Jenkins, I run my series 1 on super unleaded and use Shell V power, to which I add about half the recommended dose of STP lead replacement additive. The engine certainly better and livelier on the Shell V power and the MPG has improved too. on a completely rebuilt engine, just about run in I am getting about 22/ 23 mpg. on regular unleaded I was getting 19 or 20
These engines were designed to run on High octane petrol originally
Hope this helps, Arthur.