I owned two Volvo diesels and would use the block heaters anytime it got below 40 degrees. That equals much less wear on the engine (oil's warm), MUCH less strain on the battery, and you get almost instant heat (coolant's warm). It's one of those things that you should use often if you have safe access to an outlet for electricity. I also noticed that my windshield NEVER frosted over in the winter when the block heater was plugged in, and snow would slide right off the hood/windshield too. I wouldn't be too concerned about when to use it (i.e. temperature) - just use it. No one will fault you for using it when it's cold. I can't say enough good things about block heaters. When my 760 got totalled in an accident it had 370,000 miles on it. I swear part of that was due to plugging in the car over the winters.