The director shall adopt rules to require any retail dispenser of gasoline, whose variator price is set in liters, for every grade of gasoline sold, to post on top or on the face panel of each gasoline pump and on any other pricing advertisement in conspicuous numbers comparable in size to any other posted price, the price per United States gallon for the gasoline dispensed. If the variator price is set in gallons, no liter price posting or advertisement shall be required. Nothing in this section shall be deemed to require that gasoline be sold by the gallon.