No No No! Do not use regular motor oil or MMO as oil for foam air filters. Neither of them are sticky enough and will quickly drip out of the foam, leaving your filter dry and not filtering as well. You can use motor oil in a pinch but MMO has way too much solvent content to be used. Buy a specific foam filter oil from an auto parts store or better yet a motorcycle/ATV shop.
A specific foam filter oil will stick to the cells of the foam better, catch more dirt and not rinse out so easy if the filter gets wet some how.
Personally, I like the no-toil foam air filter care kit. It comes with filter cleaner, filter oil and filter rim grease, all of which are designed to work in concert together and all of them are good products on their own. I have also tried UNI air filter oil which is also very sticky and works well.
Both of them are available at your local motorcycle dealer or online. Also, Napa can usually order the no-toil products for you.