Doesn't amsoil say 50:1?
I use atf in both my bikes. Change it every few rides. No issues.
I use klotz. As long as you use a quality product you should be fine.
Hmmm. That sounds odd. Where the cylinders ported prior? A spacer plate is only to keep the piston and head from contacting. Never to compensate for timings
I say hook that tester up to more stuff because HF testers are known to do that.
Do a leak down test and stop running it.
Check your electrical system too