Generally, the pre-1972 SOHC4 bikes required old-style premium gas. Today the 'mid-grade' fuels (in the US) have approximately the same burn rate as those old premiums, because of the added ethanol.
Starting in 1973, the SOHC4 engines were all retuned for less octane. They ran well on old-style regular gas then. Today, it is best to test between the regular and mid-grade on a given bike to see which one causes smoother throttle response: if there is pronounced throttle lag in the low midrange area with a big throttle pull, try a higher grade of fuel to see if it helps.
