I just got back from a 140-mile round trip to the little town of Houma, LA, in the '03 Buick Park Avenue. Highway 90, smoothly paved for the most part, and nearly flat. At one point, rolling at 57 in a 55, cruise control on and windows up, A/C off, the trip computer reported 38 mpg (!). I know it always reads about 10% too high, so 34.2 is more accurate. Astonishing on a car this size.
My week-in, week-out mileage, 75% city, seems to be about 21.5, though this trip will pull the average up quite a bit.
Oil at present is Chevron Supreme dino, 5W-30, finishing up a rinse phase of Auto-Rx. Next week, I'll have PP 5W-30 put in along with a Wix filter.