Which one of the following will cause oil consumption?

Excessive main bearing clearance can lead to oil consumption because it allows more oil to escape from the bearings into the engine's crankcase. When the clearance between moving parts increases, it can disrupt the normal flow of oil, resulting in increased oil misting and splattering. This not only leads to a loss of oil but can also affect the lubrication efficiency, causing the engine to consume oil more quickly than it should.

Normal operation depends on maintaining the correct tolerances between parts to ensure that oil stays where it is needed for proper lubrication. If the main bearings are too loose, as in the case of excessive clearance, oil can bypass the necessary lubrication zones, thereby increasing oil consumption rates as the engine struggles to maintain adequate lubrication.

The other options do not directly cause oil consumption in the same way. Improper oil viscosity may affect the engine's performance and lubrication, but it typically doesn't lead to increased consumption as significantly as excessive bearing clearance. Frequent oil changes can actually help maintain optimal oil levels and quality, while poor fuel quality may harm engine performance but is less likely to affect oil consumption directly.