I have a SEARS lawn tractor with a 15.5hp b&s model 28n707.
It starts easy runs great until I engage the mower deck, then
After 1 min great billowing clouds of whit/gray smoke from the
Exhaust. If I disengage the load it stops smoking and run fine.
? is it valves, rings, govener, or what. I paid $200 for the mower
Have I been clipped.?

Hi,
I would say to change the oil to a known good quality 30W to start with (filter too, if equipped) and see what happens.
Look around the base of the head to see if there is evidence of a compression leak from the head gasket. Check the air filter area for evidence of oil entering through the crankcase breather. My guess is one of these areas is at fault if the problem continues after the oil change.
Peace,
Clarke