The lead in petrol was principally to prevent pre-ignition (also known as "pinking") and to lubricate and protect the valves and valve seats in four stroke engines. From this it follows that leaded petrol is not required for two stroke engines. We understand from a variety of sources that leaded petrol has little effect on small, single cylinder four stroke engines used in older lawn mowers because they are not run for lengthy periods and do not run fast enough, hot enough or at high enough compression for the presence of lead to be a factor. Unleaded petrol is therefore suitable for two and four stroke engines. We have heard from collectors and enthusiasts that some adjustment to the carburettor jets and ignition timing may be necessary when using modern fuels although it is unclear whether this is to do with the absence of lead or the different ratings of modern fuels compared with older equivalents. However, we also understand that there is no problem with using a lead replacement additive to replicate the performance of traditional leaded petrol.