Do I need an Oil Cooler Thermostat?

I put one on my uls for that reason and operates very well.

One of the best improvements I made to my Aircraft, oil cooler and temps are a non issue plus fast oil temp warm up.

https://www.aircraftspruce.com/catalog/eppages/xcomthermostat.php

Or just apply a strip of 2" aluminum tape across the radiator and or oil cooler. Many of us do it this way. Thermostats for me are just in the way. It's just a personal preference. You can buy a roll at any hardware store. They make a thick aluminum tape and a thin paperbacked one that you pull the paper off when applying. The thin one is the one to use. Wrap it all the way across the radiator or cooler and just around the edges a tad. Works well and when it warms up again just reach down and pull it off. It's cheaper and less intrusive under the cowl and less hose are parts to replace at the 5 year rubber change.

As Roger stated, Aluminum HVAC Foil Tape works quite well, and is simple and easy to use. 
It is also the proper tape to use on your Duct Works.  (Foil Duct Tape,  Not Cloth Duck Tape!)

Water does not need to get over the Boiling point to evaporate out of the OIL.
After a flight, check the appearance of the Oil.
Unless it looks like Brown Milk, you do not have a water problem.

Thanks everyone. I hope to have a chance in the morning to try out the tape. The thermostat may be in my long term plan. Time will tell.