I'll second Mike's comments.
Our Clebrity and Cutlass Cruiser were both very good on gas, the Celebrity with the 173/2.8 and a 3100 in the Cruiser. The Celebrity regularly got 32-33 miles a gallon on the highway; the Olds was about the same.
As to the efficiency increase with mileage, it sounds like a take on the old-fashioned Knight Sleeve Valve engine, in which as the carbon deposits increased, the engine's compression, and as a result, its gas economy.
Whether it worked well or not is a matter of debate.
Charlie Larkin