What people do with their cars has little in common with old aircraft engines.
The old guy who ran the first maintenance shop I worked in always said that the worst thing that happened to aircraft engines was the introduction of multi viscosity oils. Prior to multi viscosity oil being available, when it was too cold for an engine to start without preheat it just wouldn't start. After multi viscosity oil became available engines got easy enough to start that people got complacent with preheat. The longer I'm around these engines, the more I think he was at least partially correct.
Assuming the OP is referring to the same aircraft that has had major engine work recently completed which is being asked about in another thread, the chances of me starting a cold soaked engine is about zero regardless of what oil is in the engine.
Exactly.
Cars, once again, are not airplanes.
That aircraft engine, being aircooled, runs at much higher temperatures than your car. Therefore, it has larger clearances to avoid seizure when cold aluminum pistons warm up in still-cool steel cylinders. Aluminum expands at twice the rate of iron or steel. And since those clearances are large and the engine gets quite hot, and since any mineral oil decreases in viscosity as it warms, It has to start out thicker than the oil in your car. A common multigrade aircraft engine oil is 15W50 or 20W50, while a common automotive multigrade is 5W20. When it's at -10°F (+14°F) there's a big difference in viscosities between those two oils. Furthermore, the oil pump in an aircraft engine is well above the level of the oil in the sump, and it has to suck that thick oil up the pickup tube. Think McDonald's milkshake. It doesn't move too fast. So the engine doesn't get much oil, and those bearings resent that. In your car, that engine has to spin the flywheel and the alternator and so on. In the airplane, that engine is already having to spin that propeller, a much, much larger load than your car's engine sees right after startup. Those bearings are carrying significant loads already, and getting no lubrication, so they start burning up. Isn't that nice? No,
it's not the same as your car.
In the Aircraft Systems course I used to teach in college, one of the classes covered lubrication. The night before I would take quarts of W80, W100, and 15W50 and stick them in the freezer in the fridge. Get them down to maybe -10°C. Just before the class I would take more quarts of those same oils and immerse them in a tub of water on the stove and heat them until the water boiled and hold it there for 15 minutes or so. In the class I would take room-temp quarts of the same oils and do a pouring demonstration onto a sloped ramp. They'd see what they saw when they topped up the oil in the airplane. Then I'd take the frozen oils and do that pour demonstration, and the difference was absolutely stark. The W100 would barely move. W80 was a bit better. The 15W50 flowed not too bad, since it's near an SAE15 oil at that temp (same viscosity at -20) but you still wouldn't want to have to suck it up any tube. Then the heated oils: W100 flowed, W80 flowed faster, and 15W50 flowed about like W100, of course, since at that temp is has the same viscosity as W100, which is an SAE50 oil.
This chart is for a diesel engine 15W540 oil, but the principles of aircraft multigrades is the same:
This subject comes up every year when the weather turns cold. Every year. The forum archives are full of it.
