An air leak won't damage compression. What's your reason behind that theory?

390PSI is fantastic for a 200k mile engine. A dodge pickup truck gas engine at that mileage would be on its last legs leaking more oil than the exxon valdez. The rated PSI for compression for new OM61x motors is rated between 380-430PSI new. Your still falls in the "New" region.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k