I've tried it all, and the "barrel" heaters and "waste oil" heaters work good, but the main problem is they are high maintenance.
You have to gather wood, or hunt down waste oil, then light the fire, wait for it to heat up.....you spend more time working on the heat then working on your car. Then you have to clean them when you get done.
So for a couple of years I used a propane forced air heater, and liked it. The problem with it is, when the propane bottle gets low, or starts to freeze up (from running too long) it causes the heater to put out a lot of fumes and CO. The fumes burn your eyes and nose. The propane bottles can be a hassle to mess with too, I live 30 miles from town so swaping bottles at 11:30pm isn't fun.
NOW, I have a Kerosene/Fuel Oil forced air heater and LOVE it. What go me hooked on these is my uncle has one, a Reddy Heater.
It's got a built in thermostat, 170,000 BTUs, it turns on and off to keep a comfortable room temperature, it's low mantenance, and doesn't take up a lot of space. It can easily be stored away in the summer time.
Mine is a small cheap one. Gave $35 for it at a thrift store.
It doesn't have the features, only on or off, but it's still great.
And my big secret.....I run it on a diesel/gasoline mix. (about 60/40)