Waco YKS-6 pilot reported not shutting off the static collection can valve during pre-flight resulting in oil pressure loss during cruise flight. The pilot decided to divert to a nearby airport.

Narrative

Flight from home airfield ZZZ to ZZZ1. Going around perimeter of Class C; noticed flicker in oil pressure. I immediately recognized this is not normal and made immediate turn into Class C toward ZZZ2. After 30 sec of checks while headed to ZZZ2; [requested priority] and made immediate landing on ZZZ2 grass Runway XX. Shut down and saw that a large portion of the oil tank contents had emptied via rocker box scavenge drain that had not been closed. Pressure did not go down to an unsafe level before getting shut down. While I was pre-flighting and removing the static collection can for rocker box scavenge oil; I was distracted while removing the can and did not shut off the valve. It is on the checklist that I was using; but again I was distracted. It is obvious whether the valve is on or off and the presence of the collection can is obvious. Fortunately; flow out is fairly restricted and is not pressure oil. I believe there are adequate checks in place to prevent recurrence with the exception of needing to maintain a sterile environment during a preflight.