I know of fleets that are plugged in to timers, 30 minutes on and 30 minutes off.

I have personally plugged in power strokes during 20 degree weather and have enough heat after 20 minutes to start them hundreds of times for 25 years with like 6 trucks and have never had a engine failure.

Then I have like 10 internationals that get plugged in, same deal with them, 20 minutes and they will start in 20 degree weather.

Once a year I have to clean the male plug on the trucks with a wire brush to get rid of the corrosion and the female plug on the cord has to be replaced at least once a year because they burn up from the amps that the heaters pull which is close to 1500 watts, like having your toster plugged in 24 hours a day.

I really don't think engine wear is that big of a concern from my 25 years of experience.

We usally leave them plugged in over night, sometimes on Fri night I'll unplug them to save $$ when we are not working the weekend and plug them in 1 hour before starting time on Monday morning and they always start no problem even in the coldest weather.

And I have replaced many cords on the trucks because they get burned up after a few years.