I'm probably wrong, but in my day on the flight deck, what's now called JP-4 was Avgas. JP-5 was the jet turbine fuel of the day. It's been half a lifetime, so what do I really recall correctly?
I remember it being called Avgas.
There were legends of people obtaining some for their hot cars in the 1970’s.
If it was 50-50 diesel / gasoline (as Wiki claims) seems like you’d have to clean those cylinder heads, piston chambers and even the piston heads every so often.
Been awhile since I rebuilt a couple of engines (and also swapped out the 2V heads on my 351 Cleveland for 4V heads (larger chambers). Still low compression - 8.5 with the original heads.