I am curious to know how many GA pilots always have oxygen on board,

I don't, and suspect that most span canners don't. Oxygen starts to be
required above 12,500 in some cases, and is always required above 14,000
(unless it's a pressurized airplane). Most spam canners probably fly a
few thousand to ten thousand feet MSL.

It's been reported that for some, especially at night, altitudes above
5000 feet cause noticable impairment. Nonin makes a fingertip device
(pulse oxymeter) to measure blood oxygen saturation. I'm curious myself
as to what my readings would be. However, it's a $300 device, and I
don't often fly that high anyway.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
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