Synthetic oil can help extend the life of a high mileage cars engine.
When the engine in the car gets high mileage as in over 200,00 miles or so then the engines bearings get some wear to them as well as the piston rings.
So by adding a synthetic blend of engine oil to the engine it helps keep the engines bearings and other parts lubricated better which extends the life of the engines bearings etc.
I always switch over to high mileage engine oil once my engine gets over 150,000 miles on it and by doing so I've noticed that the engines last longer.
Of course make sure you keep the oil level at the proper level and make sure to change the engine oil every 3,000 to 10,000 miles depending on your engine.
Also change the engine oil filter along with the engine oil and your engine will last a very long time.