The 1940 price of gas at 19 cents a gallon may have been outrageous (per Vintagetvs comment below), but I remember passing through an Iowa town on a trip with my dad in about 1960.

Dad told me there was a "Gas War" going on in that town, and that automatically meant we were on the lookout for the lowest price. I spotted a station which had gas at -- are you ready? -- 19 cents a gallon! Wow.

We pulled in, and Dad told the attendant to "fill it." In those days, if you said "fill it," it was expected that the attendant would wash the windshield and check the fluid levels under-the-hood. As the attendant stepped away my dad turned to me, and I'll never forget what he said: "Just watch, at 19 cents a gallon he's not gonna wash the windshield." And sure enough, he didn't.

From the “The Conoco Collector’s Bible” comes the claim that Germ Processing was the first motor oil additive ever used by any oil company. Germ Processing was a special oiliness (polar) additive invented and patented by British scientists Wells and Southcombe in 1918. It was made from castor oil components. In 1934, Conoco developed a synthetic version called GD-160, later called MDS. This was the material referred to as “Oil Plating” on cans and in advertisements. “Germ Processed” itself was oddly chosen as the name of their first motor oil because it was a “germ of an idea.”

19 cents adjusted for inflation is $3.35, or roughly the same as today, or even a little more.