Ford OBD-I codes 39, 41, and 91?

Is anyone familiar with a good solution for these codes? I have a 1989 Ford Taurus LX with the 3.8, and found these memory codes while doing a KOEO scan. I performed this scan because my check engine light has flashed on several times recently while driving, but hasn't stayed on. Code 39 was for "trans slipping" - would a trans additive help? Codes 41 and 91 are for no oxygen switching indicating a lean condition. I've had this before, and it seems like this car goes through a set of oxygen sensor every 2 years. I had been using Bosch, then switched to NGK, but I guess that didn't solve the problem. My engine had been running rich because of a bad temp sensor, which I have replaced. The ECU thought the engine was cold, and was never turning on the fan, and also running rich, so I wonder if that ruined the sensors. I know from owning this car that I'll have these trouble codes saying the engine is running lean when in fact it is running rich. Is this just normal, or have the sensors been ruined? Fuel economy seems to be a few mpg lower than normal. Should I assume the sensors are OK if the MIL isn't on constantly?