Which system does the oil system NOT provide filtered oil to?

The correct choice is that the oil system does not provide filtered oil to the wheel brakes. This is because the wheel brake system operates independently of the engine oil system. Wheel brakes generally utilize hydraulic fluid for their operation, which is separate from the engine oil that lubricates and cools various components of the engine and its associated systems.

In contrast, the other systems mentioned, such as reduction gears, propeller, and engine bearings, are integral parts of the overall engine system and rely on engine oil for lubrication, cooling, and cleaning. The oil system specifically circulates filtered oil to these components to ensure optimal performance and longevity, as they operate under high temperatures and pressures and require efficient lubrication to prevent wear and tear. Therefore, the wheel brake system's reliance on hydraulic fluids rather than filtered oil confirms it as the correct answer to this question.