No doubt a topic with various opinions. Tanis says their pre-heat, which heats the oil pan and block separately, can be run continuously in ambient temperatures below 100F. When I started flying 30 years ago, it was commonly said that continuous pre-heat could "cause" corrosion. We were to go to the airport to turn on pre=heat the night before flight, no sooner. The logic never made sense. To have corrosion, you need moisture, which the engine makes on it's own. I would think a warm engine might evaporate that moisture, which is why I leave the dip stick out on the wing, when the Tanis are plugged in. Hoping the moist air has a place to escape. Condensation simply can not happen, from ambient air, unless the surface is below the dew point. If the engine is warmer than ambient air, condensation is impossible. What am I missing? I don't understand the 100F upper limit, which might be revealing. I suppose there could be confined passages with combustion moisture that don't evaporate, but how would heat do harm. Still Tanis seems comfortable taking on the liability of permitting continuous use.