Sorry my last post was lame. I would do several steps before taking the thermostat housing out to put tape on it. For all I know there might already be tape on there.
I'm not 100% sure that this is a leak at the thermostat. It might be that it just happens to be at the lowest so that is where the oil goes. If you have enough length in your lines you might want to rotate the plate so that the thermostat it is on top.
Another option is to call one of the bigger Mocal distributers, like
Pegasus, and see what they have to say. .