SPAL makes a little 4 inch fan that moves a ton of air. It's a puller style fan only, so it would need to be on the back side of the cooler, but it's a great fan built by a reliable high quality brand. I used one before and never had an issue with it. Only a 1.9 Amp constant with a 4 amp peak power draw as well.
Obviously, the only downside is that it costs as much as the oil cooler kits themselves, but it is waterproof, dustproof, and will be realiable.
http://www.jegs.com/i/SPAL/063/30103...ySTBoCpMvw_wcB