Just oil the side that is facing the incoming air stream. If you oil both sides, you will probably have trouble with your MAF. Too much oil on the filter and you'll get residue attaching to the MAF filament. That will, of course cause other related issues. If you've got the stock air box, then oil the side of the filter that goes down. And let it dry for a day or so after oiling it too. That will help reduce the oil getting on the MAF.