The issue would be if the oil is considered trash a the curb or an item on the owners property regardless if the oil is considered trash.

That old non running mower you are going to get around to fixing some day is still your property until is it hits the curb on trash day.

If there was a propane tank on someones property and another person drained propane out of it, that would be considered theft. If a DEA agent came across what looked to be a propane tank and took a sample looking for ammonia with out a warrant, I doubt the evidence would stand.

The van with tanks reminds me of a story where they would park over gas station tank fill covers and pump out fuel.