you will get lots of advice. lots of confusion.
I own a homebuilt aircraft (built by an old guy and his son) that is powered by a Continental O-200 engine.
I run the engine on 91 octane unleaded car fuel because I found the lead emissions an unnecessary pollutant.
I use Champion REM38E spark plugs in the engine.
I notice a slight tendency to carby icing more than I got with 100/130 octane avgas but that is all. carby heat sorts it out nicely.
I use 20.5 litres an hour of fuel. so at $1.50 a litre that is $31 per hour direct cost.