The only way to completely drain the oil in the cooler and lines is to remove the cooler and flip it upside down, as well as gravity drain the lines. Not worth the trouble at each oil change interval, in my opinion. The residual amount won't matter. When adding new oil just top up to between the H and L lines on the dipstick, as you normally would. You end up using an extra quart of oil or so compared to stock.
I'll let someone else describe the mechanism of the thermostatic plate, as I don't think I can explain it in an eloquent way.
