Pistons, rather than all propellers. Basically imagine a really old car engine, because simplicity is crucial for reliability and ease of maintenance so all those "fancy" features your car had by the 1990s aren't available, however instead of turning wheels the piston engine turns a propeller. Like really old car engines these piston engines tend to be designed for leaded fuel. Because this is relatively cheap to do, all the cheapest planes aimed at GA (General Aviation, ie you just like flying a plane, not for pay) are like this.

Propellers are a very common means to make aeroplanes work though, instead of a piston engine, which is cheap to make but relatively unreliable and expensive to run, you can use turbine engines, which run on JetA aka kerosene, and the rotary motion of the turbine drives the propeller making a turboprop. In the US you won't see that many turboprop engines for passenger service, but in the rest of the world that's a very common choice for medium distance aeroplane routes, while the turbofan planes common everywhere in the US would in most places be focused on longer distances between bigger airfields because they deliver peak efficiency when they spend longer up in the sky.

JetA, whether for a turbofan or turboprop does not have lead in it, so to a first approximation no actual $$$ commercial flights spew lead. They're bad for the climate, but they don't spew lead into the atmosphere.