Here are the three items we are using to pre-heat the water, engine oil and transmission fluid. In winter these mods add about 2 to 3 MPG to the trip total MPG and probably not that much in summer, but I still turn them on about 2 hours before leaving home and the shop. The pre-heat allows the engine to lean burn from the first mile driven and should help the engine live longer with no cold starts.
Number one item is the water heater and circulating pump that brings the water in the entire system to 125 degrees. The pump allows the water in the engine block and in the radiator to stay at 125 degrees.
Here is the 400 watt oil pan heating pad that will bring the oil up to 160 - 180 degrees in 2 hours.
Here is the transmission heating pad. It is 100 watts and allows the trans to shift smoothly from the start of the trip.