change o2 sensors?

Does anyone know or had a noticeable gain in gas mileage by changing their oxygen sensors? I recently got new ones really cheap but havent put them on yet. I have a 96 xj 4.0 with 120k miles. Im getting about 18mpg on the highway any driving like grandma. should i be able to tell a difference with new ones. Just did a tune up and someone suggested it. Any suggestions?

I gained about 1.5MPG when I changed mine at 200K miles.