I'm not sure what an oil change coupon is. I presume it is something that gets a garage to change the oil in your car?

You also haven't mentioned actually speaking to garages. If you can get a few garages on board, they might be willing to put a flyer or something in their office. It might also be attractive for the cheapest garage in any given town to advertise themselves as such.

Disclaimer: I know nothing about business.