Leaded and Unleaded petrol

[Roger the Horse]

OK, this is the most niche question possible but I really want to know because I haven't been able to find an answer anywhere and it's bothering me that this question is lingering in my head for no particular reason.

We've all heard how running a leaded petrol engine on unleaded can cause knocking and other potential engine issues further down the line, but I've never really heard what would happen if one tried to run an unleaded engine on leaded.

So, what would happen if one did?

Along similar lines, how far back in time would one be able to take a typical modern petrol car and fill it up with petrol it would run on at the road side? I assume eventually either the quality of the fuel would drop too low or the car wouldn't like the leaded fuel or there wouldn't be anywhere to fill it up, but when, and why? How would this be different in the US to in Europe? I'm curious.

 

[Tornado]

It would clog the fuel lines/injection system and almost immediately destroy the catalytic converter.

 

[Slash]

Lead was used as lubricating/seating agent for the valve train. This is why to run unleaded on a leaded engine the valve seats need to be hardened or the valves will not seat in the chamber properly causing blow back and back firing from detonation. The knocking would come from that. Rest of the fuel system will be fine. My dads engine in his truck orinally was designed for leaded.