Piston aircraft engines use a pressurised lubrication system to deliver oil to the most heavily loaded and critical components. The main bearings and crankshaft are among the highest-loaded components in the engine because they transmit the full combustion forces from the connecting rods to the output shaft. Lubrication of these bearings is provided through a combination of two methods working together.

The first method is high-pressure oil delivery through internal oil galleries and passages drilled through the crankshaft and engine block. A gear-type pressure pump driven by the engine draws oil from the sump and forces it under high pressure through these passages directly to the bearing surfaces, ensuring a continuous pressurised oil film that prevents metal-to-metal contact. 

The second method is splash lubrication, which results from oil thrown off by the rotating crankshaft and connecting rods as they move through the oil mist and residual oil in the crankcase. Splash lubrication supplements the direct pressurised supply, coating adjacent surfaces and providing lubrication to components not directly served by pressurised oil passages. The combination of high-pressure directed lubrication and splash ensures comprehensive protection of all crankshaft and bearing surfaces.

By both low-pressure oil and splash lubrication → INCORRECT. The pressurised oil supplied to main bearings and crankshaft is HIGH-pressure oil, not low-pressure. The oil pump in a piston engine generates significant pressure to force oil through the narrow internal passages and bearing clearances. 

Only low-pressure oil → INCORRECT. This option misidentifies the pressure as low rather than high, and it omits the splash lubrication component entirely. Main bearings require pressurised oil delivery specifically because their loading is too high and their clearances too tight for splash lubrication alone to provide adequate protection. 

By both high-pressure oil and splash lubrication → CORRECT. This correctly identifies both the pressure level of the directed oil supply (high pressure, delivered by the oil pump through internal galleries) and the supplementary splash lubrication from oil thrown off the rotating assembly within the crankcase. Together these two methods ensure reliable and continuous lubrication of the crankshaft main bearings throughout all engine operating conditions.

Only by splash lubrication → INCORRECT. Splash lubrication alone is completely insufficient for main crankshaft bearings, which operate under very high loading from combustion forces. Without a pressurised oil film in the bearing clearance, metal-to-metal contact would occur rapidly at engine speed, causing catastrophic bearing failure within seconds to minutes of operation.