You have options: What is the Best Ethanol Fuel Treatment? (at http://www.fuelsystemguide.com/what-is-the-best-ethanol-fuel-treatment/ ) Ethanol is an octane booster, so if you choose an additive that doesn't include an octane booster, you might consider a separate one. However, that might not an issue with the Slant 6. When your car was new, regular was 93 octane, but the formula for determining octane has since changed, and the current 87 octane is the equivalent of the 93 octane during the 1960's. Most stations had only regular and premium gasoline, but Sunoco stations had "Custom Blending", with up to 8 separate octane ratings. The Slant 6 engines might have performed adequately with their economy blend, which was lower than 93. Anyway, lead was an octane booster that was eventually phased out of gasoline. You can buy a cheap container of octane booster at Dollar Tree. Xylene is also an octane booster; you can buy a larger container of it at hardware stores, which should last several fill-ups. Another option is to find a gas station that sells ethanol-free gas, which is usually about 30 to 50 cents per gallon higher than E10. These stations are more common in some parts of the country than others; Oklahoma has a lot of them, but Texas has far fewer. Speaking of lead, it was also a valve lubricant. During the 1970s, manufactures switched to hard seats so that no-lead gas could be used in them. If the head in your engine was converted to hard seats, you should be ok, but if not, you might also consider a lead substitute. You should still be ok if you drive the car at mostly city speeds without hard acceleration.