O2 sensor voltage output

[Mark Romans]

Hello all:   I have been tuning my Vette for a while using a tech1 for
snapshot and trying to get the 02 sensor to at least 800mv at wot.   I have
the MT program for my 87 Vette and I keep adding fuel trying to get the 02
sensor voltage higher up.  Lately it seems to want to go leaner.  700mv
range.  It isn't detonating,  I pulled timing out to try and eliminate that
too.  I'm running a total of 31 deg now,  stock prom is 36 deg total.  Could
the 02 sensor be contaminated with high temp silicone from the exh gaskets?
Do these things just get tired?  It still has the stock exh system so the
location of the 02 sensor hasn't changed.   Any suggestions/answers?
Thanks.    Mark.