^+1 location dosent really make a difference. How old is the o2 sensor itself? if it has alot of time on it or time spent under extremely rich conditions, it could be going bad. Low to mid 12's are pefectly safe under lower boost levels <25psi. You will normally make the most power when the afr's are in the low 12's.(under boost conditions) 15's while crusing are ideal for maximum mpg. While not under boost conditions, it is nearly impossible to hurt your motor. It will break if things aren't right under bost conditions.