I don’t know how to test an O2 sensor, maybe one of the many pros who hang out here do.
A thermostat stuck open will cause a cold running engine. If you have a temp gayge and it is reading the way it always did, your thermostat is fine. Most temp gauges run just below to just barely over half.
I am continue to work on it - the last refill sounded promising - out of 246.8 miles, 40 miles were freeway driving and still gave me 23.53 mpg.
I did fewer cold start consisting of about 3 miles total trip (used a different car for that - this vintage car frantically revs and become fully warmed up)
I was also less soft on the gas pedal when warming up - I can feel the AT hits a critical point and continue to rev if I don;t push the gas pedal a little harder - when I do it advance the gear and revs drop
in any case, the first 20 mpg occurred soon after I checked the timing - revving at 3k rpm for 3 minutes or so is capable of reducing mpg from 23.5 to 20?
Will be getting a ELM327 and an android app to see what I get through OBDII.