thanks for the suggestions - good call w/ the penetrating oil - i put
a little on, wait 20 min, then carefully worked the sensor free - it
came loose quite readily, and no apparent damage to the manifold.  i
didn't need a breaker bar (the wrench was about 22 cm long).  have one
question at the end of my write-up :

the old sensor threads look grubby, so i was concerned about the
female threads in the manifold being damaged.  i dried the penetrating
oil from the manifold, put anti-sieze on the sensor, and applied it
finger-tight, which happened in about 2-3 turns.  it seemed to be
sitting in the threads ok, but the sensor clearly was not secured all
the way.  So i carefully worked the sensor into the threads by going
back/forth about 1/8 turn each time until, now, the washer is nearly
pinched tight between the manifold and the sensor after more than 5
turns.  each time, it would get tight, then after working it, i could
get it further in.  i noticed something like a gouge in the manifold
surface outside of where the threads are during this process, but
does'nt seem to be increasing in size as if stress cuased a split.
was probably there before.  i drove around a while and things are
stable.

if i tighten the sensor all the way down, will this increase the risk
of the manifold splitting over time?   i also figure that the best
ways to see the result of a new sensor is in MPG and emissions -
right?   idle RPM, performance seem the same as before.

-bryan