Interesting. Did you use a CO2 detecter and did it apply to all Navajo aeroplanes, or just that individual aeroplane?
In those days I was routinely flying friends piston a/c, but the biggest was a Twin Com. No problems on those. I didn't fly another Navajo.
There was a simple detector in the a/c, (nil result) and an application made to borrow a sophisticated bit of kit from the RAF. I had departed the scene before it became available.
I was still doing sports in those days and was quite fit. I also had a very high peak-flow reading at CME, so I was very concerned about the strange feelings. In the bad old days I routinely used to fly the DC3 at FL130 and I was fine.
One doctor even suggested the beat frequency of the props should be allowed to increase to check for hypnotic effects. (my words not his, but that was the science behind it.) No change.
One other pilot who noticed a slight effect, was packed off to the RAF and had an EEG. Nothing out of kilter.