Thermostat in 450

[RMAC]

I had the engine rebuild in my 450 (diesel) last year, it has about 3 hours on it now.
When I use it for a short time the temp gauge goes to red and pretty much stays there, even if I let it idle for a while, it has a 180 thermostat which I replaced with another 180 degree thermostat, it appeared as if the old thermostat had a small amount of gasket cement in the springs. The machine now seems to be running cooler according to the gauge but still stays on the high side. My question is can I replace the thermostat with a 165 (the one sold for a gas engine) for the summer and change back to the higher temp thermostat in the winter or would I damage the motor? also could the problem be just that the engine hasn't broken in yet.
Thanks