Synthetic oil can go for 10k miles of regular driving. Especially when you have 8 quarts in the crankcase. But that's not the issue. All modern oils have many additives. They keep your engine clean, fight corrosion etc. All additives in a 4-5 quart system are burned out of the oil in 5k miles. So I'm thinking it's not the oil that will break down but the additives. Just another thing to think about.