I often feel using synthetic would be a total waste is some vehicles.
The best oil in the world won't stop contaminants from getting in, filters aging... Quality conventional oils also have improved dramatically over the years.
Consider someone who should change oil by time, not mileage. Say they only drive 10-12K miles a year and have few long trips to really dry things out. That is "severe service" of a different kind and there may be a cost benefit ration that favors more frequent changes with conventional oils.
gerry