Cessna 172 (trainer plane, not known for efficiency) burns 32 l (8.5 lb) per hour at a cruising speed of 122 knots (140 mph, 226 kph). Assuming no wind, it gives mileage of 14 l / 100 km (16.5 miles per gallon).
Taking into account that there are much less planes than cars, they emit 0.13% of all the CO2, emitted by transportation, comparing to 81% of motor vehicles (623 times less) [1]. So I would say their lead emissions are too low to bother (and mostly harm their owners who are close to the exhausts). Would be nicer, of course, to use unleaded, but not a big deal to lobby for legislations.
[1] https://communicatescience.com/zoonomian/tag/cessna-172/