'92 190E O2 sensor

I need to replace the O2 sensor, poor gas mileage, soft sooty tailpipe deposits, plus not being changed at the 60,000 mile interval. I was told that the sensor in the '90 5.0 Mustang was exactly the same. (except for the 3 wire connector that must be changed) Ford dealer (parts) knew nothing of this, and in fact asked which one? seems there are 2 sensors for this model. one costs 75.oo the other 86.oo MB dealer cost for O2 sensor 186.oo Does any one know which is which? numbers,size,? .......................Rick