I copied this from Quakerstate.com
Myth #7
Synthetic oil lasts longer than conventional oil or extends the oil drain interval.
You can go longer (i.e., 25000 miles) with synthetic oils.
Using synthetic motor oils can double your oil drain interval.
Synthetic motor oils can be used twice as long as conventional motor oils.
Changing only your oil filter every 3000 miles and topping off the oil can extend drain intervals.
Fact
Quaker State® does not recommend extending oil drain intervals beyond the “severe service” maintenance interval of three months or 3,000 miles, whichever comes first. Also, we are unaware of any automobile manufacturer in the United States that currently recognizes using any synthetic oil beyond the recommended oil change intervals outlined in their owner’s manual. We emphasize severe service since the majority of motor vehicles are operated in severe driving conditions such as short trips (under 10 miles), dusty or sandy conditions, cold weather, extended idling periods, trailer towing or other harsh conditions. Under ideal conditions, however, such as a dust-free climate, highway driving, light loads, perfect engine performance, etc., the oil drain interval may be extended to the vehicle manufacturer’s recommended “normal service” period (generally between 3,000 to 7,500 miles). Contamination by normal wear particles, water, fuel, and other combustion by-products, as well as additive depletion, are the main reasons for changing conventional oils on a regular basis. Synthetic oils are equally susceptible to this problem. The only way to remove these contaminants is to change the oil and filter within manufacturers’ recommended intervals
Myth #7
Synthetic oil lasts longer than conventional oil or extends the oil drain interval.
You can go longer (i.e., 25000 miles) with synthetic oils.
Using synthetic motor oils can double your oil drain interval.
Synthetic motor oils can be used twice as long as conventional motor oils.
Changing only your oil filter every 3000 miles and topping off the oil can extend drain intervals.
Fact
Quaker State® does not recommend extending oil drain intervals beyond the “severe service” maintenance interval of three months or 3,000 miles, whichever comes first. Also, we are unaware of any automobile manufacturer in the United States that currently recognizes using any synthetic oil beyond the recommended oil change intervals outlined in their owner’s manual. We emphasize severe service since the majority of motor vehicles are operated in severe driving conditions such as short trips (under 10 miles), dusty or sandy conditions, cold weather, extended idling periods, trailer towing or other harsh conditions. Under ideal conditions, however, such as a dust-free climate, highway driving, light loads, perfect engine performance, etc., the oil drain interval may be extended to the vehicle manufacturer’s recommended “normal service” period (generally between 3,000 to 7,500 miles). Contamination by normal wear particles, water, fuel, and other combustion by-products, as well as additive depletion, are the main reasons for changing conventional oils on a regular basis. Synthetic oils are equally susceptible to this problem. The only way to remove these contaminants is to change the oil and filter within manufacturers’ recommended intervals
