if your lambda is dead and fools ECU giving a lean condition, you have a lot of fuel.
this is just my personal thinking what could be:
not burned extra fuel is washing oil from your cylinder walls, gets through the rings and gets into the oil sump, rings has no oil to ride on, so fuel leak accelerates, your cylinder walls/rings can overheat, this is a bad thing.
strange if you have no warning light and exhaust smoke is normal..
do you feel any missfires at idle or acceleration?
i would check:
all cylinder compression (dry & wet!)
lambda working condition/parameters.
fuel injector can be leaking (not O rings, but injector itself)