Valvoline Instant Oil Change [VIOC]

We went to Valvoline to get our oil change a few weeks ago. Valvoline overfilled the oil tank and it leaked into our engine. We didn't know this until we were driving our car away from Valvoline down the highway when the engine began to smoke. Valvoline did tow our vehicle back to our mechanic. After inspection from our mechanic, we found we now need a new engine. Our car was running perfectly before getting this oil change. We found to put a used engine in is going to cost $4,500. Our car is an older 2004 Toyota Camry so Valvoline said they would give us $3,200 since that is what the car is estimated at. We feel this is unfair. Number one, this vehicle has been paid off and number two, there is no way we can find a comparable vehicle for $3,200. Valvoline has been giving us the runaround for last few weeks, i.e. Manager is not here, we have to check with someone higher up, etc. No one calls us back and we call everyday but only get excuses. For the last two weeks, we have had to rent a vehicle. Valvoline I feel is responsible for putting a new motor in or giving us $4,500 so that we can look for a new vehicle. I would like answers NOW! This is ridiculous!