What indicates worn rings when a small amount of oil is placed in the cylinder and compression improves?

When a small amount of oil is introduced into the cylinder and it results in improved compression, this is a clear indicator of worn rings. In an engine, the piston rings create a seal between the piston and the cylinder wall, which is essential for maintaining compression. When these rings become worn or damaged, they can no longer effectively seal the combustion chamber, leading to a loss of compression.

By adding oil, you temporarily enhance the seal because oil can fill in the gaps created by worn rings. This restoration of compression suggests that the primary issue lies with the rings themselves, rather than with other components like the exhaust valve or the piston. Therefore, the observation of improved compression upon the introduction of oil serves as a diagnostic tool indicating that the rings are indeed worn and need to be replaced for optimal engine performance.