my car gradually is decreasing MPG

[GasSavers_RoadWarrior]

Is the tranny slipping or something? DOHC and auto together aren't the best combo, but sounds like it should be doing better than that. "Very" slowly is sometimes counterproductive with an auto, gotta be fairly brisk so you can get out of the fluid thrashy wasty range, you want midrange acceleration, give it 2500 rpm or so. Also have a suspicion that your valves could be all carboned up, and that your rings are stuck down with carbon...

Personally, I'd do the following if it was my car, i) give it a clean out dose, 8oz isopropyl alcohol, 4oz laquer thinner and 4oz varsol in a full tank of gas, beat the snot out of it every day for a week and feed it a pint of water through the vacuum tubes every time you got it hot. ii) change the tranny fluid and put "Lucas transmission fix" in the refill, (Leave the rest of the snakeoil alone) iii) Refill the oil with 5W30 Valvoline Maxlife (very high detergent stuff) do the following for the change, drain 2 quarts, put in one quart ATF and one quart varsol and run it for 20 mins, drain that, then put the maxlife in it... you might find maxlife smokes like hell for 2 weeks and turns black real quick... let it... after 2 months change to Castrol or Pennzoil high mileage oil, they have less harsh detergent and better seal conditioners, so if it's still a bit seepy and smoky by that time, it should stop with those.

After that I'd maybe change the O2, get NGK or NTK ... but not while all sorts of crap is going to come flying at it while you're cleaning stuff out.

Edit: BTW when you say you "upgraded" spark plugs, what did you use? Not sure if Saturns are Bosch haters or not, they might not be since it was derived from an Opel engine.