This is a great way to pull a prank on one person, an entire parking garage or a neighborhood.
Take ordinary white granulated sugar and place pour it around the gas cap. spill a little on the ground and make it look obvious. You do not want to put a single grain in the tank. This prank works on the principal of fear, not destruction.
When your target sees the sugar, they will immediately suspect that some unscrupulous person poured sugar in their gas tank!
The beauty about this prank is no physical damage occurs to victims car. The worst thing that happens is that they get a tune-up, which in turn will cause their car to run better.
Pour the sugar around random cars in the parking lot at work. Then send out an e-mail that you found sugar around your gas tank and enquire if anyone else noticed sugar around there cars.
Sit back and watch the fur fly.