What was found
DURING CLIMB OUT APPROACHING FL180, THE PF NOTICED THE OIL PRESSURE ON THE LEFT ENGINE FLUCTUATING PLUS OR MINUS 6 PSI (36-42PSI). CONTINUED THE CLIMB WHILE MONITORING, THE OIL PRESSURE CONTINUED TO FLUCTUATE BUT WORSENED AS WE REACHED OUR CRUISE OF FL330. WE CONTINUED TO MONITOR AND WERE DISCUSSING WHERE WE MAY GO IF NEEDED. THE LOWEST IT GOT WAS 26PSI WHILE FLUCTUATING UP TO 30. THEN WE DECLARED THE EMERGENCY AND GOT DIRECT TO KSDF. THROUGH THE DESCENT OIL PRESSURE SOMEWHAT STABILIZED AND WITH LOWER ALTITUDE AND POWER SETTING WE REACHED A STABLE 33-35 PSI. WE LANDED WITH NO ISSUES. AFTER LANDING DOING THE WALK AROUND, A SIGNIFICANT LEAK FROM THE LEFT ENGINE WAS FOUND. MX INVESTIGATION FOUND OIL CAP NOT REINSTALLED AFTER ENGINE OIL SERVICING.