Your air filter (the foam type) sits on top of your carby in it's metal casing - I have B&S model 98902 - and I usually clean mine very throughly in hot soapy water and leave it dry out overnight. Before replacing, it need to be oiled extremely lightly so as to keep out the dust. Use a special foam filter oil and work it in well. Your hand should be greasy rather than oily. This type of oil is also used in air filters on motorbikes. DON"T do as I did first time up and saturate the filter as your mower will choke for air and you'll spend a long time cleaning out the carby.
When you put it back in the metal casing make sure that the foam filter acts as a gasket between the two metal parts of the casing. Like all things, filters do not last forever and it is wise to replace them every couple of years.
Boogie of Holland Park Qld
December 2007