Non-jet engine aircrafts run on avgas, which isn't diesel like jetfuel, basically just high octane gasoline.

Nitpick: A minority of general aviation planes have diesel engines.

Huh? There are approximately zero amateur pilots that own jet aircraft -- they cost too much.

Certainly not enough to help keep alive the market for diesel pickup trucks.

According to a 2017 report by the Airline Owners and Pilots Association (AOPA) [1], general aviation -- defined as all civilian flying except scheduled passenger airline service -- consumed 209 million gallons of avgas and 1.8 billion gallons of jet fuel.

NB: turboprop aircraft also consume jet fuel.

[1] http://download.aopa.org/hr/Report_on_General_Aviation_Trend...

Air cargo is probably the majority of that 1.8 billion gallons. Then there are chartered aircraft and aircraft owned by corporations and other wealthy organizations. The helicopters operated by news organizations. The aircraft used to supply areas where ships and trucks cannot go, e.g., Antarctica. The vast majority of the aircraft I just described are operated by professional pilots.

A turboprop engine will cost millions of dollars in purchase cost and maintenance cost over its lifetime just like other jet engines will.

The more I read this document, the more confused I get. I am most curious about the proportion of this jet fuel being burned by helicopters. The document has some information about rotorcraft, confirming that helicopters are included (though rotorcraft also includes petrol-burning gyroplanes...)

Page 5 for instance lists rotorcraft. But page 5 is confusing to me; has the categories 'single engine', 'multi-engine', 'turboprop', 'turbojet', 'rotorcraft', etc. Are aircraft being double-counted in these categories? Is a single-engine turboprop plane counted twice, in both the turboprop and single-engine categories? Is a Bell 206, a multi-engine turboshaft helicopter, counted as multi-engine, rotorcraft... and also turbojet/turboprop? There is no category on this page for turboshaft aircraft. Maybe the nature of these classifications is clear to a pilot, but they aren't to me.

Page 7 breaks down total operations into the categories jet/piston/turboprop, and seems to have piston operations as about 1/3rd of the total. 1/3rd is a far cry from 1/9 ratio suggested by 209 million gallons of avgas vs 1.8 billion gallons of jet fuel. Here is my suspicion: that difference comes from turboshaft helicopters. Typical turboshaft helicopters burn many more gallons of fuel per hour than other forms of GA aircraft.

Also a nitpick: GP mentioned amateur pilots, but many general aviation pilots are doing it professionally. I would wager most helicopter-hours are probably being flown by professional pilots. For instance, all the helicopters being flown for news stations and police departments have professional pilots and fall under general aviation.