'97 lumina here, 3.1. Same gas mileage as you. Drive 17 miles to work (34 round trip) - all highway, cruise locked at 73 mph. only get 23 mpg
new plugs, wires, fuel filter. PCV valve rattles, so I assume thats good. Good tire pressure, not sure why i have poor mileage. air filter is good too. Had the head gaskets replaced, and they resurfaced the heads and hot tanked them. also (according to the receipt) they checked and adjusted ignition timing if necessary. I was hoping for a huge increase in mileage, but it made no difference.
I have 127k on it. I DO NOT recommend a flush. I would just drop the pan, replace the filter, and replinish the fluid that was lost. I have read on these forums that a flush can cause more harm than good.