Although not a Diesel (Which run cooler) on a petrol to make sure it warms up faster I never have the heater turned on till its at a decent temperature as it sucks up quite a bit more fuel. Annoyingly I could run it on LPG from cold and not have to have it run rich. Which is a bit annoying because lpg doesn't condense on the cylinder walls like petrol does when cold.
Anyway Thermostats are pretty simple devices. As you imagine they always fluctuate depending on Load / Revs / Speed and outside temperature. Also how hot you have the heating.
I've always thought about fitting a webastro heater to the car to warm up the coolant and inside of the car before use. Ok this doesn't warm up the oil but least the car would A) be warm B) more efficient from the off.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts