Heads-up to X35d people with a Shell Gasoline card
 

FYI, the Shell Gasoline rebate that the Shell card provides is, quite precisely, a gasoline rebate -- Shell does not give the higher rebate if you buy Diesel.

So in my case, I get a larger fuel rebate at a Shell station if I use my non-Shell credit card.

Thanks! I would've never guessed it. Time to look for a new card. Perhaps switching to Chevron.