I own a dealership. We use Costco gas whenever possible because it's always the cheapest and a Top Tier brand. With the Costco card we get an extra 4% off.
Once every couple of months I do a 35 gallon purchase at Kroger. We save about 80 cents on fuel that way and the net price drops below Costco. I have a unique credit card (Alliant) which gives me 2.5% off everything for the first $10,000 a month.
My business is already pretty complicated when it comes to tracking expenses so I just use that credit card. That cost me a little over $100 in lost savings but it makes my life a lot easier when I do my taxes.