Its quite simple, a command(or maybe could be a function using alt-gr) to refuel your car by 10% for an increased price ,f.e 5x the price of the fuel station.
it can be called something like "Use fuel jerrycan". Personally i run out of fuel alot and i end up getting stranded, even when i use /mech i mostly get no response at all. Also, i have at least 5 vehicles with 0 fuel and i cannot use them at all. Since when i spawn them i cannot drive to a fuel station so my only option is calling a mechanic which as i said above only works 1% of the time or having to get a towtruck which is a pain in the ass.
I don't think this needs a poll but upvote if you support it, i guess...
EDIT: I think 10 Liters of fuel is enough to drive to an actual fuel station.