Worn piston rings, bad valve seals,
The piston rings may be worn out .
Piston rings are worn.
worn piston rings
worn piston rings or barrels
Either worn valve guides, worn rings, or a holed or cracked piston.
If there is oil getting to the top of the piston and burning then you have a blown head gasket or the piston rings are worn or broken. Engine must be disassembled.
Your piston rings need replaced. One of the two piston rings on a piston is for oil control. This is either worn or broken and needs replaced.
Worn piston rings, Burned valves, Sticky valves,
Combustion pressure getting past the piston into the crankcase. Worn rings or failed piston are normal causes.
Worn piston rings will cause your car to burn oil. It will smoke, and you'll have to add oil frequently. Additionally, the performance will fall off as compression is lost past the worn rings. There may be a "slapping" sound as the piston skirts slap the cylinder walls, and this will occur in cases of advanced ring wear.
Just from normal friction and vibration. This may be more pronounced in worn cylinders or worn piston ring grooves. If the cylinder compression is ok, then it shouldn't be a problem.