air con doesn't run oil, in runs gas (compressed into liquid form). and if I were you I'd just get a $50 second hand oil cooler from a wreckers, as every air conditioning condenser I've seen is too big to be an oil cooler, and I'd be willing to bet you'd end up spending more modding an air-con condenser
There is oil in your aircon line... mixed in with the refridgerant.
The condenser isn't there to cool the oil in the aircon system though its there to cool the low pressure side of your aircon system before it re-enters your compressor.
I think you will find that it is not economically viable to modify the condenser to use it as an oil cooler.
Add to that it is designed for far thinner liquid (oil + refridgerant) which is far cleaner than your crankcase oil would be after a couple of thousand kms... I would think that crankcase oil would clog it up quite quickly.
Actually, it cools the gas on the high pressure side of the system into a liquid form before it enters the low side of the system via the metering device. What you are referring to is the evaporator which is responsible for removing heat content from the desired area. But thats not the point.
I doubt that the condensor would be suitable(as others have stated). Even though there is oil mixed with the refrigerant its almost like water, it has pretty low viscosity.