As many stops as you can buying the minimum $40 and accumulating the discount, then use the discount to fill the car up from as close to dead empty as you are comfortable to let it get is the best saving.

The saving is in cents/litre, so the less litres you buy to get the discount, and the more the litres you buy when redeeming the discount the better off you are.

All assuming you resist the temptation of overpriced candy bars and sodas etc. when you do all those $40 purchases,otherwise the evil empire wins.