If we're getting all scientific about this, does anyone know the heat capacity of engine oil. We always put a 1kW fan heater inside the cowling and under our 0-200 for 30 minutes before a very cold start but I would be surprised if it had much effect on oil temperature.
We use it to warm the carburettor and induction system so the petrol vapour stands some chance of getting up to the cylinders before condensing (no primer, just an accelerator pump). With sufficient sucking in it always starts immediately after being in an unheated hangar at about -5C. Sometimes it stops again just as the oil pressure is coming up and, although it restarts with a couple more primes, I've never understood why.