The only way to know for sure if your oil can handle extended drain intervals is to send a sample to a lab for analysis. The lab will provide a break-down of the oil's condition (additives, chemical composition, etc.) and what the oil contains (contaminates, metals, fluids, etc.). A search of the forum or web using search term "oil analysis" will locate several labs that can analyze your oil.
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1977 300D: 300,000+ miles
American Honda: Factory Trained Technician & Honor Grad.
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