On Feb 2, 3:52 pm, wrote:
We've started our airplanes at ambient temps as low as -25C (-13F)
without preheat, but they sure don't like it.

I'm surprised you got by with that. Must have been a Continental
engine? Lycomings have no crankshaft feature that will sling oil to
the camshaft lobes & most oil pump output has to flow over the relief
valve which has no access to the heat of the engine. It would be
interesting to find out how long it takes a genuine oil fog to
develop, My guess is quite a while.

Another factor is how long an engine has been sitting. A couple of
days is one thing, but if it has been two weeks a few more drops of
camshaft oil would have drained such that the need for preheat has to
be greater.

Having summer 50 weight (100W) oil instead of winter (say SAE 20
weight) is about the same as a 40 degF temperature penalty.

That's my take on it.