… we no longer only get "Not Quite Gas" during the winter months, we get it all year ‘round."
That's another thing that I have suspected. Modern cars, with the fuel pumps in the gas tanks, supposedly can't get vapor lock, but my previous car, with the pump in the tank, on hot summer days, not infrequently stalled and had to be towed. Symptoms consistent with vapor lock, as an M.D. might say. Caused by fuel which evaporates too easily, I believe.
That car, a 1990 Geo Prizm, originally got 30-35 mpg. When they started oxygenating the gas, sometimes with MTBE, sometimes with ethanol, the mileage went down. In later years, it got mileage in the high twenties. That was probably in part because I was driving more locally, but not entirely.