
Most cars have drain petcocks in the radiators, and I've never 
seen nor heard of a vandal opening one.  I imagine that there
would be an even lower risk with an oil plug because you have 
to crawl furthur under the car to open it.  

Car vandals are usually real traditional in their methods, and do things
that don't get them dirty, like keying your car, dumping sand, sugar
or mothballs in the gas tank, TPing it, etc.
