I have some problem with new on-off butterfly valve which provide zero leake(API598) by metal to metal sealing but this mention valve fail acceptance test in my workshop hence I contact valve's supervisor to solve this issue. He disassembly the valve and check all things such as alinement,  clearance and others.
    For closure test (Seat leakage test) he apply Molykote  D 321 R to coat sealing surface therefore this valve able to perform zero leak.
    According to API598 "Valve inspection and testing" mention that the sealing surface shall clean and fee from oil, grease and sealant.If necessary to prevent galling, the sealing surfaces may be coated with a film or oil that not heavier than kerosene"

Using Molykote with sealing surface is comply with API598 or not?

No, the valve does not met API-598.  THe valve may still be OK for your needs, but if it needs Molykote on the seat to meet the leak rate, then it does not meet the requirement that "the sealing surface shall be clean and free from oil, grease and sealant."  Molykote on metal seated valves is pretty common, and it does help the valve seal tighter in the factory test.  However, once installed in the plant, the Molykote can wash out of the valve and it will leak more than the factory test.     

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