'90 LX 5.0 Poor Mileage, Ox Sensor?

My old girl used to get over 300 miles per tank, has been for years (I've had the car since 1990 when I bought it). Over the last few months I noticed that I'm only getting about 260-280 miles per tank using the same gas and driving the same route each day (I use it mainly for going to and from work; most of which is interstate).

A buddy of mine suggested that the cause might be the Oxygen sensors. I checked tire pressure and the air filter, both are good. How can I test the Oxygen sensors? What else could I check?

Thanks.

I hate to be a downer, but I think your engine is just experiencing old age. Parts are wearing, perhaps compression isn't what it used to be. May need some Caragra(Viagra for cars)