This again is a complex topic. I'll just to share information with you so that you can study up on it.
Different engines use a variety of seals—such as valve stem seals, cam seals, and crankshaft seals—to keep oil where it belongs. When these seals harden, shrink, or wear out, oil can seep into the combustion chamber or drip externally. Modern synthetic oils can sometimes expose weaknesses in older seals because of their different flow characteristics.
Engine configuration affects oil control, crankcase pressure, and how oil returns to the sump. Flat (boxer) engines, for example, naturally hold oil in the cylinder head, which can encourage oil seepage past valve seals. Inline engines typically control oil better, while V-engines vary based on PCV design, head angle, and oil return paths.
The oil control (scraper) ring removes excess oil from the cylinder walls during each stroke, keeping only a micro-thin layer for lubrication. If this ring becomes clogged with carbon, loses tension, or wears down, it can allow significant oil to enter the combustion chamber. This is one of the most common—and most expensive—causes of oil burning.
The PCV system manages internal crankcase pressure and recirculates oil vapors back into the intake. A clogged PCV valve can cause excessive pressure, pushing oil past seals, rings, and gaskets. Conversely, a stuck-open PCV valve can pull too much oil vapor into the intake, leading to increased consumption.
Modern engines often use low-tension rings to reduce friction and improve fuel economy. The downside is that they don’t scrape oil as aggressively off the cylinder walls, which can lead to higher oil consumption—especially as the engine ages. This design is one of the major contributors to oil-burning issues in newer vehicles.
Turbochargers are lubricated and cooled by engine oil, and their seals are exposed to extremely high heat and pressure. If a turbo’s internal seal begins to fail, oil can enter the intake or exhaust side, resulting in rapid oil consumption and visible smoke. Turbo engines therefore tend to be more sensitive to oil quality and viscosity breakdown.
These seals prevent oil from dripping down the valve guides into the combustion chamber. Over time, they can harden or crack, especially during extended heat cycles. Worn valve stem seals typically cause oil burning most noticeable during startup or deceleration.
As oil loses viscosity due to heat, shear forces, or extended service intervals, it becomes easier for the oil to slip past rings and seals. Thin or degraded oil can also vaporize more readily, contributing to consumption without obvious leaks. Engines that rely on low-viscosity oils (0W-20, 5W-20) are more susceptible to this issue.