1988 300 SEL, 208K mi., no major engine work in past. Engine runs great. Consumes 1qt/500mi. No smoke, I've tried to rev in a garage to see if it makes blue smoke and doesn't. Put front end on ramps, changed transmission fluid. Drove on the interstate the next day, BILLOWING blue smoke from exhaust for about 10 miles, then slowly went completely away. Engine never skipped a beat at 70 mph.

Any suggestions?