HI...I was filling up the Chevy 235 3100 with a medium grade gasoline. Out comes the manager from the store (76) and he tells me that because of the lousy gas here in N. California I should only use a supreme grade of gasoline in "my collectors vehicle"(not a racing fuel like 100 octane) OR I should mix half and half of medium and supreme. I said thanks and I will do that next time???? OK, I don't think I have harden valve seats(this is a 1957 engine)he said the supreme grade would be cleaner burning and better for the plugs and valves?? Was/is he correct? I've done the technical reading but what about you who have driven with a mix or straight supreme is / was he correct? Thanks, Steve