Originally Posted By: Cujet
FYI, aircraft operators in very cold climates used to dilute the oil with fuel intentionally. This reduces viscosity for easy starting in severe cold. The fuel evaporates rather quickly after start, and oil pressure and temperatures remain normal.
In fact, some aircraft had a built in system, from the factory, that added fuel to the oil for cold weather starting. The requirement was that the oil reach 100 degrees F prior to takeoff.
First of all, an airplane engine is not an automotive engine. It is a mistake to assume they are similar enough that various conditions or systems are interchangeable.
The airplane engines that did use fuel to dilute engine oil have sumps that are much, much larger than a typical automobile, or at least a typical post-WWII automobile. As in sump capacity is measured in gallons, not quarts.
Aviation oils in general are higher viscosity than automotive oils, for example Aeroshell 100 is equivalent to SAE 50 automotive viscosity.
Aviation oils until relatively recently had no winter rated oils and no multigrade oils that were legal to use, so using fuel dilution was not a 'good practice", it was the only possible practice to adjust viscosity.
By far the more common method of starting in cold weather is to tarp up the nacelle and provide heat under the tarp. The use of the equivalent of a campfire was hardly unheard of. The automotive equivalent is to tarp up the engine compartment and run exhaust from another running vehicle under the tarp to heat up the engine / sump, ground clearance being what it is making open fires difficult.
If you've never had to do either to an aircraft or a vehicle, you haven't been in a cold start situation that would benefit from oil dilution via fuel either.