I assume this was inserted in quarts (like plastic discs that Gulf used) to be found by the attendant when he added oil to a customers car. He would then remove from the can or funnel and redeem towards a gift or promotion from the company. The incentive was for the attendants to ask customers if they could check the oil, and then sell/add the quart(s) right there at the pump.

Anyone have any similar ones from other oil companies?