I have an uncle who works on high end cars exclusively. He has a small shop that he runs off his property in a large pole barn.
He has a deal with the local napa to get all the parts delivered right to his building.
The guys with the high end import cars don't mind paying $9 per quart for Mobil 1, or $6 per quart for Castrol GTX.
I don't see you making any money on a $18 oil change unless your labor is free, and you don't have any costs to run the building. Unless you are getting an upsell on every customer.
There are some dealerships here that will do a 10 or 15 dollar oil change, but they make it up by up-selling on a ton of other services that people buy. You need a lot of volume to make money doing it this way.
We even have a toyota dealer here that runs months ever year where they do free oil changes on toyota. No thanks.