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I've read that block heaters are best when on consistently at a lower setting in terms of engine wear. This seems supported by intuition, in that the same amount of heat over a longer time will result in a better distribution of heat through the engine. The thermal conductivity of Fe also drops as the temperature increases, which could exacerbate this. Since automatics tend to rev higher when cold, presumably to heat up the rings and avoid excessive wear and oil contamination, I wonder how effective a block heater would be at reducing wear given the ECU would think the engine is warmer than it actually is and the warmup portion would be somewhat retarded. Manuals of course are in the hands of the driver at anything other than idle.
Also, I think a significant portion of fuel consumption at startup involves a rich mixture lighting off the catalytic converter/s, and I wonder how much fuel block heaters save compared to cold. I imagine a car with a single pre-cat oxygen sensor would likely realize a greater decrease in consumption compared to a car with a pre and post-cat oxygen sensor, so testing in that case would likely be fruitless, but testing the second system could provide useful info as to how much fuel consumption is associated with the higher idle, and how much is associated with lighting off the emissions system.
In any event, it seems that less over more is better than more over less for engine wear, but I imagine both are better than nothing in the cold.
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