I have a 14.5hp B&S powered SEAR LAWN TRACTOR, it starts easy and runs fine until
I engage the mower deck :ie UNDER LOAD, within a minute or two it starts smoking very
Badly.
Upon inspection I find OIL in the Air filter plenum, if I wipe it dry and restart, it repeats.
Is this caused by leaking valves, worn rings,or some engine adjustment.

Hi,
Could be a bad breather. Something is pressurizing the crankcase above normal. First I think I would check for a blown head gasket, one leaking into the crankcase but not into the atmosphere.
Peace,
Clarke