The cooler is really only needed on the track. rpm's heat
oil.
On the street, you really do need a thermostat. When the
oil is cold, the thermostat will pass about 10% of the
oil through the cooler. That allows all of the oil to warm up at the same rate. Otherwise, the valve would open and you'de get a big slug of cold oil. That's a bad thing.
When the oil is hot, it only passes about 90% of the oil through the cooler. That little bit of bypass helps prevent pressure drop.
I use a Russell billit thermostat that opens around 180*. In the winter I cover the cooler with duct tap to block air flow. If it's less than 45-50* and I'm on the highway, I still have a tough time getting the oil much above 140*.
I'm working on a plan to alleviate this problem for good. Stay tuned.