Take the standard tactic of writing a letter expressing your misgivings and stating that if the engine of the car has any problems within 6 years that could be attributable to this lack of oil on delivery you hold the dealer wholly responsible. State that you give the dealer the opportunity to accept these terms or take the car back and refund your money immediately. Send the letter by Post office Special Delivery directly to the dealer principal of the dealership. Keep a copy and staple the certificate of posting to it and keep it with the car's documents in your house. What has happened then becomes a matter of record.