Probably defective valve stem oil seals allowing oil to leak along the valve guides and enter the combustion chamber/exhaust system.
Valve stem oil seals tend to be more affected by time than mileage and some of the materials used have been shown to be adversely affected by some of the oil additives used by oil companies.
One further thing to note - inside the modern engine a large volume of oil is delivered to the top of the engine (valve gear) every minute and in order for the oil level around the valve gear to remain constant the oil must drain back into the oil pan at least as quickly as is delivered. A too high level can overwhelm even good valve stem seals. Sometimes the oil return ports can become restricted...
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Probably defective valve stem oil seals allowing oil to leak along the valve guides and enter the combustion chamber/exhaust system.
Valve stem oil seals tend to be more affected by time than mileage and some of the materials used have been shown to be adversely affected by some of the oil additives used by oil companies.
One further thing to note - inside the modern engine a large volume of oil is delivered to the top of the engine (valve gear) every minute and in order for the oil level around the valve gear to remain constant the oil must drain back into the oil pan at least as quickly as is delivered. A too high level can overwhelm even good valve stem seals.
Sometimes the oil return ports can become restricted...