there are three types of air cleaners- oiled foam, oiled fabric and dry paper. oiled foam filters rely on the oil aspect of the foam, which is the actual filter. the foam must be oiled on these kind of air cleaners. air filter oil can be used as well as 30w engine oil. 90w gear oil also works well with this type of air cleaner. fabric filters utilize a surgical-like material within the metal mesh of the air cleaner. fabric filters can be long lasting when properly maintained. the third type of air cleaner is the paper filters, which are most popular on street motorcycles.