You want the engine oil temperature to come up quickly, the advantage to using a cooler in the radiator is that your engine thermostat will help that along. I am plumbing an external air cooled oil cooler on my car but it has a 205F thermostat that bypasses 90% of the flow when the temperature is below 205F and has its own electric fan that has a 205F switch as well. The 10% that runs through the cooler at low temperatures to to help keep the temperature of the oil in that loop about the same as the main loop so when the switch occurs it doesn't pass a slug of cold oil into the engine.
