Ok, I am in just a bit warmer climate than you, winter solstice yesterday and its a sunny 22c today.
I can not see any reason to have an oil thermostat, at least here in Australia, with stock 20b cooler I could only use about 50% power before cooking the oil with a 13b.
Can't really comment about flying with cold oil. I would count that as a total victory if ever in that situation.
Aftermarket remote oil thermostats are available if you need one in the system.
I tried a 75mm thick aircond condenser as I had a couple of new ones that were originally going to be used for coolant, better but not good enough. still seeing 235-240f on partial throttle takeoffs, 175f on descent. no thermostat, but possibly too much internal restriction as predicted by Steve B.
This weekends project is to add more sensors before cooler just to possibly confirm the inevitable need to purchase & import a fluidyne. local cooler mfrs have been absolutely no help.