Your best source of oil change interval recommendation and type (API specification) and "weight" of oil is your owners' manual for each specific vehicle.
If you follow it, you are unlikely to have any engine problems related to insufficient engine oil maintenance.
Typical synthetics have the same additive package as mineral based oils for each API category. The primary advantage of synthetics is longer service life because the base stock oil is more resistant to oxidation. Sythethics can also be an advantage in arctic-like conditions because equivalent grades flow better at extremely low temperature.
Virtually all models where the original factory fill is synthetic and synthetic is specifically recommended for service have on-board oil quality monitoring systems. The monitor recommends changes based on driving conditions and some also have an oil quality sensor. The monitors typically reach "zero life" somewhere between 5-15K miles. Lots of cold starts and stop and go driving in hot weather will consume oil life much faster than steady freewas cruising in mild temperatures.
Even with such systems there is also usually a time limit regardless of mileage or the oil quality monitor, and one year is typical.
Last edited by Duke2.6; 12-15-2003 at 01:27 PM.