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.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.