Interesting. As you said,the engine will knock. As you know, the octane rating for diesel is very low...that's why compression ignites it. I don't know why folks would say this increases mileage. When you say you wouldn't try it on newer vehicles, I assume you're talking about the risk of killing the catalytic converter. My 2019 Mazda adjusts timing on the fly, constantly keeping the engine on "the verge of knock." When I run high test, I get an additional 25hp at a certain RPM range. I wonder how the car would react to diesel.