There are a wide variety of severe conditions, such as stop and go driving, hauling a load, short trips, extreme hot or cold temperatures and dusty areas. These conditions really push the oil to the limit. People in sales who spend a lot of time on the road or highway commuters may qualify. But if you have any doubts, don't wait to change your oil. Changing oil is a lot easier than changing an engine, and more economical.
Synthetics extend drain intervals.
Synthetics are superior. The superior protection is from the processing required to manufacture the base stocks and the different additive packages they require. Despite better performance, synthetics need to be changed at regular intervals. The additives wear out, just like in conventional motor oils, and need to be changed. Changing the filter is great, but you've got to change the oil along with it, given that the oil's additives wear out and contaminants get into the oil itself.