1995 Oldsmobile Achieva Excessive Oil Burning

I own a 1995 Achieva, bought used, with 140,000 miles. That was 2 years ago, now it has 161,000 miles. Have changed the oil about every 3,000 miles, add injector cleaner about every 3 fill-ups, and recently have added "Engine Restore". It looks like blue paint, but did seem to help recover lost horsepower in part. I just drove the car over 1,600 miles, all Interstate, and went through over a gallon of oil, by the time I had returned, and it was full before I left. Why is it using so much? The fastest I have drove it is 70 mph.