Depends on how sneaky they are being with their definition of 'gross profit'. Traditionally gross profit is revenue minus the cost of goods sold. On that basis, it would be impossible to run a business with a gross profit of 6.7% (£40 on £600), especially one that has to deliver a heavy product like oil to a customer.
Operating profit (can also end up being referred to as PBIT – Profit before interest & tax or simply Net Profit, although these terms can often get differentiated), which is gross profit minus operating expenses, might be 6.7%.
To illustrate if you look at all industries and calculate the avereage gross profit margins for all businesses the figure is 9%. Obviously the spreak is pretty big, anything from 1-2% for supermarkets all the way up to 25%+ for some specialist consulting firms.
I would say Gross Profit should be something more like 25-30% but don't forget that they will be thinking operating profit as they've already delivered it to you and therefore will be looking to recoup that cost.