The filters K&N sells are made from layers of woven cotton, not paper. Their oil is compatible with alot of filter types, but paper filters should not be oiled, since oil can break down the material in paper filters.
You can however do as I do and use K&N filter oil on the prefilter that is around the paper filter.
The hard part is being able to see if you have a good quality foam or paper filter, since there are dozens and dozens of grades of foam, paper, cotton gauze, etc. There are also dozens of types of filter oil that are compatible.
The key is using a thicker, stickier filter oil with the lower grade filter materials and a thinner oil can be used with higher quality materials.
Just remember, the less you filter the air, the more power, which is how a K&N filter works, since the better the air is filtered, the more restriction there is. In fact in the K&N instructions it says not to clean the filter too often, since a slightly dirty filter cleans the air better.........