Two stroke engines are designed to be used with a mixture of petrol and oil. Manufacturers specify the correct ratio of petrol to oil in the engine operating manuals. There is some evidence that modern fuels when used in older engines may require slightly lower amounts of oil than the original recommendation. Too much oil may suppress proper combustion which can hinder starting and effective running. Many enthusiasts find that a bit of experimentation is required to find a suitable petrol:oil ratio using modern fuels and lubricants.