It's not a bad thing to show appreciation or to spread some kindness to people. Who knows what they're going through? You might just make their day, or even restore their faith in humanity or even God. Sometimes people just need even the smallest sign that someone cares.

Ha ha, "they're not going to siphon out the gas and give it back." So true.

I think you said it's a diesel truck, so hopefully you didn't put gas in it. :P