If the car's a keeper then rebuild. 250-260psi is getting low and that much pressure mismatch is a bearing killer. If not a keeper then I agree with Ken300D and sell or re-ring #5 to buy time before selling. Strange that the engine doesn't drink oil. Kind of sounds like either a stuck ring or rings that have worked around to where the openings are slightly aligned, or a bad taper in #5. All provide a path when cold. Plugging it in (if you have a block heater) might buy you some time. Have you tried comparing hot and cold compression on #5?

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daBenz - 1970 220D