A-Series engines were originally designed for Leaded fuel. As leaded fuel is burned, it leaves behind a deposit on the combustion chamber areas, most notably the valve seats on both valves, heads, and valve stems. This deposit builds up over increasing mileage to a point where after some ten's of thousands of miles a decent layer has formed - hence the term 'lead memory'. This makes it perfectly feasible to run on unleaded fuel only without any problems for many miles before the severely abrasive nature of unleaded fuel totally removes it.