When fitting a new air filter, or if you have washed your existing filter out with fuel, you will need to add some air filter oil to it to stop the fine partivces of dust and grit damaging the engine. The best way of doing this is to place the filter foam into a small plastic bag together with a few drops of oil and massage the oil into the foam.
A drop of After Run oil should be placed into the engine ,and the engine turned over a couple of times to ensure distribution. This should be done when the engine is cool, and when you dont intend to use the engine for a period of time. It keeps the engine lubricated, and stops any left over fuel congealing.