I've never used it, but the list they give is just to provide examples and is by no means exhaustive. Chances are if you get fuel there it will be coded to qualify for the category.
That said, Discover does list the following exclusion:
Purchases made at gas stations affiliated with superstores, supermarkets and warehouse clubs may not be eligible.
I also did a spot check in my account of gas stations I've used and all I found were listed with the merchant category "Automated Fuel Dispensers." You may be able to call customer service and ask, but I think the only way to find out for sure is to test it.