Here's an important point to consider about engine temperature. That is the fact that the engine must warm up enough to get rid of the blow-by gases produced by normal combustion. This "vaporizing" of the contaminites helps to keep the proper pH of the oil.
In cold winters like we get in Wisconsin, the contaminated oil may get so bad with raw gas that the bearings are actually rinsed of any lubrication. I have changed spun bearings out of several engines where I have first drained out an extra quart of oil. The extra quart was not oil. And the oil was more like dirty gasoline than oil.
I will have to say that engines with carburetors and a bad or poorly adjusted choke are far more likely to have problems than engines with fuel injectors.
Sludge (mixture of water and oil) can also form due to low engine temperature. In the colder climates you may notice sludge as a creamy white to brown color goo in the valve cover or on the dipstick. Either the vehicle is not being run long enough to properly warm up or the cooling system has a problem warming up enough.
Hope I'm not being to long winded!
