A good rough indicator is that the flame on a gas burner should be pure blue. If you see yellow tips, or a yellow flame, it's not getting enough oxygen for complete combustion.
the carbon monoxide can kill you pretty fucking dead tho
Measured at the downpipe with the sniffer in the lambda sensor hole it shows no (or almost no) CO and a bit more NOx than you'd really want, but after the cat there's no measurable NOx either. The tailpipe emits basically just warm carbon dioxide and water vapour.
What is the downside to this? Obviously availability and ease of filling is part of it, but any other downsides?
In theory the engine makes a bit less power flat out, but I never drive flat out. It's quieter, runs cooler and cleaner, and has a bit more low end torque because propane burns with a bigger slower bang.