"Oil dilution switch"?

I was reading the flight manual for the P51 Mustang (I was actually trying to find out the flap settings) and came across this in the description of the oil system:

"The oil system uses standard Air Force oil dilution equipment. This allows the oil to be thinned with gasoline to make the engine easier to start in ambient temperatures below 40°F. Thinning the oil requires allowing the engine to idle with the coolant flap open until the oil temperature drops to 50°C or less. Then, before stopping the engine, oil is diluted using the Dilution switch on the Engine Control panel of the front dash. This will dilute the oil until the engine is ready to be started again. Once the engine warms up, the gasoline in the oil is quickly evaporated."

Aviation engine oil was not multi grade so was very thick when cold and diluting it makes sense for starting but it does add weight to the mandatory instruction to ensure the oil reaches a set temperature before full power is used!

I bet they don't use the oil dilution switch now!

As spirit evaporates off, all it would do was to flush the oil from the cylinder walls, cranks and bearings - surely? One the aircraft had been standing for a while, the spirit in the oil in the sump (did they have sumps, or tanks?) would evap off, leaving the poor mechanics with a thick treacle to try to warm up, so the pump could pump it?
Standard op procedure in the far north/south is to drain the oil off any engine, into a can, which can be hung over a heater (fire) before you want to run the engine, then the already hot oil would be poured in.

Cant see this being wrong if the flight manual says this is what happened, however no doubt Andy can clarify this when he reads the thread .

Garry

I have so far no found a technical description of exactly how (or how much) gasoline was added.
As it is an electrical switch I presume it ran a pump.
The Merlin was a dry sump engine so I guess the object was to thin the oil in the bearings and on the cylinder walls so there was less oil drag when cranking the engine at the next start rather than to dilute the entire oil content. The thick oil in the tank would still pump but at low volume so an extended idle would be required to reach the minimum oil tank temperature.
The manual did say that below 20 Fahrenheit external heat had to be applied to the engine oil to start the engine.

On the same lines most cars in Canada have built in sump heaters which are plugged into the mains each evening during the winter.