Contact your attorney and tell him or her that you want to sue. Have them draft up a petition/complaint and get it on file as soon as possible. Do not pass go. Do not collect $200. Businesses simply DO NOT respond to letters, even very sternly-worded ones on important-looking letterhead. They do respond to lawsuits because they have court-imposed and enforced deadlines. They also will have to pay for a local attorney in the appropriate jurisdiction to read themselves into the file and prepare an answer (among other things).

Fees may be an issue, but there should be a consumer protection statute in your state that would allow for the attorney to claim fees (and hopefully recover them). You may even be able to find a lawyer to take this on a contingent fee basis. Chances are there is a plaintiff's firm/lawyer in your area that specializes in suing car dealers and/or manufacturers. Your lawyer will also want to check whatever service contract, invoice, documentation, etc. you signed at the time of the original oil change. It's possible that the dealer has some sort of arbitration provision snuck into their boilerplate form. That could prove problematic, but wouldn't stop me from moving forward with filing in state court.

If it wasn't clear before, it should be abundantly so by now--you WILL NOT see any tangible satisfaction from this dealership unless and until they are required to answer for their abhorrent, incompetent, and potentially fraudulent behavior in court (or arbitration). You seem to have an incredibly forgiving personality, which has no doubt contributed to and emboldened this dealer to act in the manner that is has. The time for contrition and collaboration needs to be over. Your car is gone forever. You need to become comfortable with the idea of receiving a check and not the vehicle you drove on the lot that day. And you need to try to make that check as big as possible.

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-"Wax on, right hand. Wax off, left hand. Wax on, wax off. Breathe in through nose, out of mouth. Wax on, wax off. Don't forget to breathe, very important." -Kesuke Miyagi