@LawnMonkey 

That's up to you based on how you want to see it on your reports.  If you want to see it as a separate line item on your P&L, create an expense account called 'Fuel Expense'.  If you don't want to track it separately from other job supplies like oil, trimmer string, etc., you can use 'Materials and Supplies' or something more generic.  There's no right or wrong because it's a legitimate cost.  It just depends on how much detail you want on your reports.