From what you write, you would seem to have a good case against them: when a mechanic, repair shop, etc. is negligent, or unreasonably careless, in how service is performed, he or or it will be liable for the damage or costs that are thereby caused. Not replacing the oil and filter is clearly negligent; you therefore should have a good case against them for the repair cost, up to and including the cost of a new engine. You may also be able to get other related costs--e.g. towing; renting a car while yours is inoperable; etc. You could sue in small claims court as long as the amount you are seeking is less than the small claims maximum.