The general SOP for operations like WalMart is to try and get you to give up by dragging it out. The key is to let them know you're not going to give up and go away. That you can write letters to the local paper, picket their store on sidewalk public property, and cause them all kinds of bad press. Send them a registered, certified letter with photocopies of the receipt for the oil change and all documents related to the engine failure due to their negligence. State in the letter you want full satisfaction within 10 days of receipt of the letter. If 10 days go by and they do not respond, initiate the letters to the editor and picketing. With the registered letter, they can't say they didn't know about it. You'll have the name of who signed for it.