How long does a fully synthetic engine oil can be used in cars?

I'm using right now the Castrol Formula RS engine oil.How long does a fully synthetic engine oil can be used in cars?
It still depends on what kind of driving you do. If it is stop and go city driving, figure 6000-7500 miles. If it is all highway miles, you can go as long as 15,000 miles between oil changes.How long does a fully synthetic engine oil can be used in cars?
I've been running my synthetic oil for 5,000 miles according to the owners manual..It comes from the factor with synthetic oil.
Normally by using fully synthetic oil you can extend the time between oil changes from 3000 miles to 5000 miles.
Look, synthetic oil is just oil. Yes, its designed to not break down as soon as regular oil, or a synthetic blend, but the oil breaking down is not why you should change your oil. As the oil sits in and circulates through your engine, its gets dirty, and builds up sludge, especially if you do alot of short trips and stop and go traffic. My personal opinion: It does not matter if you are using Regular 10w 30 Pennzoil, or 5w 30 BG Full Synthetic, every 3 months or 3000 miles, change your oil.
Ya know how helium leaks outta a balloon alot faster than air? Syn. oil also has a smaller molecular structure. It is a heck of a better oil,,,,, BUT, if your engine is well past it's 'break in' stage,you may have seepage, of oil,past the seals. Good Luck
7500 miles is recomended by most carmakers