I'm not convinced that synthetics are superior from a financial standpoint. They have advantages over dino
oil in operation (mostly, better viscosity range and resistance to breakdown), but nothing stops them from accumulating sediment, acids, water and general goop.
There are three reasons to change your
oil. One is to replace
oil that's worn out through breakdown. Two is to get particulates out of the system. Third is to get toxic goop out of the system. Synthetics in light-duty street engines can tolerate longer change intervals because of the resistance to breakdown and most engines' tolerance for crud. I don't think I'd consider extending the oil change interval in a high-performance, old-tech engine no matter what oil I used - so I'd rather use good dino oil and change at 3-5,000 or once a year rather than "save money" by using more expensive syns and tripling the change interval.