Diesel thread drift:
We've got red and green diesel at the pumps. The red is for equipment and the green is intended for road vehicles.
While there's a difference in the price (I haven't studied it, but I'm pretty sure the difference is the tax rate), I'm fairly sure that the green stuff is supposed to be of the low-sulphur variety. This is California, after all.
The only reason I noticed this was the Bobcat was running out of fuel over the weekend. Perhaps someone else has some specifics. So my guess is that your pals are running the red (equipment) fuel in the green (highway) vehicle.
Aside: when I was a kid we'd buy 1,000 gallons of heating oil for the Kenworth to avoid highway excise tax.
This drift and slightly babblish post brought to you by the smoky wonders of Lagavulin.