Over the past several years, I have answered hundreds of questions about engines: "Why does my engine ping? detonate? have its valves stick in their guides? have badly worn exhaust valve seats? Run hot? Make less power than it used to?"
The biggest problem engines face today is the fuel. It is unstable because of the additives: gasohol, ethanol, alcohol, etc., which cause fuel to oxygenate. When the fuel sits in the gas tank for seven to 10 days, it separates and accumulates water, in turn leaning out the fuel mixture.
Ever since the introduction of unleaded fuel, we lost tetraethyl lead, which lubricated the upper engine parts, and we lost octane, which used to eliminate pinging, detonation, upper cylinder head and engine wear. We now have to go back to square one to correct the problems.
The octane rating on the pump is not what goes into the gas tank