The question is, do you want to know the temperature of the
oil in the pan after it has gone through the engine or after it has gone through the cooler and before it goes into the engine? I think a pan measurement is best.
Oil takes much longer to rise than the coolant and a street engine with a 7 or 8 quart pan probably doesn't need a cooler. The engine likes the
oil temp to be around 200+ degrees, if it's not getting there, running a cooler is a determent.