Since propane or LPG/CNG is a vapor when introduced into the airstream into the engine, the engine/vehicle can be run at any angle that still allows for oil to reach the pump. Use a dry sump oiling system, and who cares if the rig is upside-down, it'll still run.
Propane/LPG/CNG has a lower heating value than gasoline, so it has less power per unit volume, but it has a higher resistance to detonation, so it has a higher "octane" level than gasoline, allowing the ignition to be advanced further.
Propane is well known for close to complete combustion with low levels of NOx, so EGR is not necessary. Better combustion also means there is less unburnt fuel making it into the oil, so the oil remains clean longer.
I have personally seen a late 90's Dodge truck with a 5.9L 12-valve Cummins deisel engine that was converted to run on CNG. The engine had lower compression from either a set of shorter rods or a special head or gasket(I don't recall exactly what was done to it), and the head had the diesel injectors replaced by spark plugs. I think it was still turbocharged/intercooled. The guy claimed to go 10kmi between oil changes, and showed me the inside of the oil cap, and it was as clean as the day it was made, and the truck had hundreds of thousands of miles on it, so there shoulda been some sort of junk in there by then, even if it was a fresher than stock engine.
I have never fueled any of my vehicles with propane or LPG/CNG, but I want to.