The major difference between a car with 300,000 miles on it using conventional oil, and one that used synthetic oil, will be found in the glove compartments holding the receipts. Totaling up those numbers will give the OP the answer. Chances are the rest of the car will begin it slow degradation, before the motor gives up. Does it matter if the transmission fails before the motor does?. On a high mileage car, the cost to repair of replace the trans will probably outweigh the value of the car. Electrical problems that cost a fortune to trouble shoot, or rust on the frame and body will make most want to get rid of the money pit. If today's oils are better then what was available 10 years ago, and if you can get synthetic oil on sale, go for it. But if you can find conventional oil that meets your cars spec for less, don't loose any sleep about using it.,,