"Griesel" is apparently another term for biodiesel, more specifically for vegetable oil used as diesel fuel.

I first encountered the term in a story submitted to SH; did a web search, and came up with a few instances of it online, such as a CNN transcript:

Ah, golden vegetable oil. It's not diesel, it's griesel.

Interestingly, apparently there's a guy whose birth name is Ernest Griesel who uses the stuff. Makes me wonder whether the term came from his name, or whether the pun was developed independently by someone else as well.