Could be any number of things, most of which are non-fatal. More than likely just loaded up from lack of "use"...something to consider is a plug change. Not necessarily the heat range (not a bad idea), but a "brand" change. Mo and I played around with spark plugs while on the dyno, and while AC's may look good, there are better choices...go Autolite, or even better NGK. It could also be how your engine is put together. Our car will "fog" on decelleration in the traps...too much oil in the top end of the engine. But, it puts out 400+ horsepower at the wheels, so I can deal with it...
I wouldn't sweat the smoke...it more than likely did the car some good to FINALLY stretch it's legs!
