Oil Coolers

[4129R]

The only reason I fitted an oil cooler to my Cooper S many years ago was the sump guard restricted the airflow around the gearbox, which is in the sump, and finned like an air cooled engine, so the sump guard made the gearbox hotter.

[HelpMyLotus]

It was recommended to add a thermostat to the oil line. You want the oil to get to temperature, but don't want it to overheat. Leaving the oil flow constant through the cooler caused it to stay below functional temperature. 

The best of both worlds.