forgett the oiltemperature, look the cylinderheadtemperature.
the two gauges behind your right shoulder, above the fuelcockwheels.

operating temp is 200-250 degrees. and if you get higher than 5000ft, engaging carburetor heating helps running the engines smooth.

I have carb heat on from about 5/6000ft. And oil temperature is very important. Most WW2 aircraft engines run at about 80-90'c oil temp, the engine in your car will as well. The difference is moden engines use multigrade oils that do not thicken up as much as the ww2 oils did when they were cold. Thats why engines were run up periodically through the night when the weather was very cold. The thicker oil would damage the oil pumps as the pressure went through the roof when you tried to pump it. Also if you follow the pilots notes for the blenheim they clearly state that the engines should not run hotter than 210 on the heads and 80-90 on the oil. These figures are not possible in game at the moment. On the BR20, which is also radial powered, these figures are easy to obtain. as a result i think the blenheim engines are incorrectly modelled in the way i mention in my previous post.