I experienced something like this with my 1967 Malibu SS. While I was at my first year of grad school, my dad kept my car. He drove it at about 25 mph for 5 mile trips, never took it on the freeway, never got it warmed up. When I got the car back it sucked oil as fast as gasoline, and belched white/brown smoke on moderate acceleration. I figured that it had major problems, so what the hell, and floored it on the freeway on ramp. The view in the rear view mirror looked like I had blown the engine, but the car seemed to run better! Did this every day for a week, and each time it spewed less smoke and ran better. Finally, no smoke, no more oil consumption. I hypothesized that the engine was very carboned up. I was later told I was lucky, that I could have caused major problems with my "on ramp" repairs, but never had it fully explained. Good luck!
87 300E, 65k miles