A diverter valve has some advantages, but also has some disadvantages. A thermostat will flow about 10% of the
oil through the cooler when cold. As it opens, that relationship gradually changes to 90% to the cooler, and 10% back to the engine. That allows all of the
oil in the system to gradually warm up to the same temp. If it was completly close when cold, then when it opened your engine would get a big slug of cold stiff
oil.
The disadvantage is that you get some cooling action all the time, and that can be bad in the winter.
A manual diverter valve is a pretty good idea, but requires some planning. If it's all the way closed, you'll get good oil heating in the winter. But you can't just open it until the engine cools down again. You would have to gradually open it as the engine is running, just like an automatic thermostat would.
I installed a billit thermostat from Earls. Simple, thought free operation.