does anyone know the heat capacity of engine oil
Don't know about yours but an IO540 is about 160kg total weight, excluding externally mounted accessories and the exhaust. Of this, the oil is only about 8kg.
The
crankshaft is probably the heaviest single item, at about (I guess) 20kg. The rest is all the various bits; some are ally, some are brass, and probably most of the weight is steel.
Taking a very average
specific heat capacity as 0.7 (kJ/kg/K) this means that 100W (actually transferred into the engine mass) would make its temperature rise by 3C per hour.
I hope I got that calculation right
Obviously it ignores heat losses from the engine, which will start to occur as soon as its temperature rises above ambient.
That is quite slow, but it probably represents the top end of what can be achieved by poking a heated probe down the dipstick hole.
If the dipstick heaters damage the oil they must be of crap design. On my engine, with 9 qt of oil, the bottom 3"-4" of the dipstick is immersed. If the dipstick hole is in a large pool of oil, then convection will easily carry away 100W, with a temp rise of the order of 10C in the immediate vicinity of the heater. If the dipstick hole is tight around the dipstick then heat will be efficiently transferred into the crankcase by conduction through the layer of oil. The important thing would be for the heater to be temperature controlled (which I guess none of them are) and to heat only the fully immersed portion. The most worrying thing would be if the heater breaks off; the engine will then have to be dismantled to get the bits out
And 100W might be what actually ends up in the engine from a 1kW fan heater poked up the cowling.
So to do it effectively, from really low temps like -10C or colder, one needs a lot more than 100W.