oil change high mileage on 2003 Chevrolet Monte Carlo

i go to NTB (national tire and battery) to get my oil change and i always get the cheap oil, and they always recommend that get the expensive one because i have high miles on the car (89,xxx) would it really matter if i got it or not and yes i do want this car to last a long time but i need to know for sure if it really matters, the expensive oil is the Synthetic Motor Oil, the cheap one is the conventional motor oil

I started using full synthetic oil about 10 years ago, my last car got 336,000 miles on the engine with out ANY major repair needed. I actually sold the car for $750 because it still ran well enough to pass the State Emissions test. So yes, I am a firm believer. I use Mobil 1 full synthetic and change it every 4000.