Your stuck with your 30 hr rebuilds no matter what happens (unless you buzz the engine and get a yellow dash light) so whilst running the pre heater might save you somewhere around 30min of total run time over that period of time, if you are that concerned then you are going to have to manually move the car from the pits to dummy grid, shut down on the grid, etc. From what I have seen, the rebuild times might be on the short side for the engines in the Radicals, The engine in the Speads has done more than that with no lose of performance or cylinder pressures.