First I clean the air filter with regular soap and water and let it dry. Then I place the filter in a zip lock bag and apply a few drops of oil and use the bad to push it in. If it is not enough to coat the whole filter, I add a couple more drops. Rinse and repeat until it is covered. Just don't use too much.
I made a trip to hobby town today and picked up a few things. I got the filter oil and am pleased with the price for how much I got. I also picked up a pack of filters which are already oiled up and ready for action. They were 3 for $10. You get the primary and secondary filters. I now have 5 filter sets to take with me wherever I decide to go.