[sammyg2]

My wife sometimes asks me a question and when I answer she says, no that's not it.



Engines do not run without fuel. For an engine to diesel after it is shut off, it needs a continued source of fuel (and source of ignition).

Back in the olden days some carbureted cars would diesel after getting shut off because the fuel in the float bowls would boil and over-flow into the manifold, or the float level was set to high to begin with.

No matter how well your car was running before, there was a symptom that was covered up by the higher octane fuel and uncovered by the lower octane fuel.
An injector can work just fine during operation and still leak when shut off.

You can switch back to the higher octane and ignore the problem or fix it.
But a leaking injector can eventually dilute the oil with gasoline.

If i were you and you're glad I'm not, I'd dump a bottle of techron in the tank and see if that makes and difference.