It is possible to make the oil "too cool". The biggest problem caused by this is condensation. You need to get your oil at least to water's boiling point somewhere in the engine, or condensation can occur in the crank case and make your internals rust. Keep in mind your thermostat is lower than the boiling point of water. Aside from the coolant keeping your engine cool... because they're all aluminum, Honda engines don't really generate or hold in heat very much anyway. At least not like my DSM does with a cast iron block. I can shut my GSX off after driving 10 minutes to work and it's still above the cold line when I break for lunch 3 1/2 hours later. Hondas cool off in like 15 minutes. You guys envy my heater in the winter.

But if you do find the need for an external oil cooler, NASCAR teams change their car's complete setup for each track, and they don't re-use their oil coolers. Usually the car builders (like Hendrick) donate them to charities and collectors that auction them off on eBay. You can get them really cheap... The model I bought... supposedly from Jeff Gordon's ride was $70 shipped, and it sells for almost $400 new. You probably wouldn't need a big one, or one as large as the one I bought.

Unless you live in a harsh southern climate (I don't know where you live) Raiden's right, you probably wont need it.