Beginning in 1989, EPA required gasoline to meet volatility standards to decrease evaporative emissions of gasoline in the summer months when ozone levels are typically at their highest. In the early 1990s, EPA began monitoring the winter oxygenated fuels program implemented by the states to help control emissions of carbon monoxide during the winter months, and established the reformulated gasoline (RFG) program to reduce emissions of smog-forming and toxic pollutants.
All you want to know about todays gasolines are on this website. They continually stress that performance is not degraded yet everywhere they tell of reduced volatility to meet emissions standards. Now tell me that todays gas is as good as it used to be. When these old tractors were designed and set up fuels were indeed different. Todays RFG is different and I'm not sure it performs better as they say.