According to specifications, what oil grade should be used after oil consumption has stabilized?

The correct choice is based on the recommended lubricating oil specifications for certain aircraft engines, particularly in the context of aviation applications.

MIL-L-22851 refers to a military specification defining a specific type of aviation oil designed for use in aircraft engines. This specification ensures that the oil meets exacting performance standards, especially in varying temperature conditions and operational stresses that such engines experience. Using oil that adheres to this specification will provide the necessary lubrication and protection needed for engine longevity and performance.

Turning to the other options, while SAE grades such as 20W-50 and 15W-40 are commonly used in automotive contexts and may also be suitable for some light aircraft under certain conditions, they do not specifically indicate compliance with aviation standards like MIL-L-22851 does. API SM is a designation that relates to oil performance standards for various automotive engines but is not specific to the aviation requirements needed for Cessna 152 and similar aircraft. Each of these alternatives may serve a purpose, but only MIL-L-22851 is tailored to the aviation context in which stability of oil consumption is a concern. This ensures optimal performance in flight operations.