What was found
WHILE ENROUTE AT 39000 FT, NOTICED RT ENGINE OIL PRESSURE TURN YELLOW & BOUNCE FROM 55 TO 57 PSI FOR APPROX 5 SECONDS. DROPPED TO 53-55 PSI FOR APPROX 5 TO 10 SECONDS. PRESSURE DROPPED TO 48 PSI. SHUTDOWN RT ENGINE. SINGLE ENGINE LANDING WAS MADE. OIL WAS OVER ENGINE COWLING AND A 3 FT PUDDLE OF OIL UNDER ENGINE. INSPECTED ENGINE & FOUND OIL RESERVOIR CAP NOT INSTALLED & LAYING ON BOTTOM OF COWLING. INSPECTED OIL RESERVOIR CAP, CLEANED ENGINE, COWLING & SURROUNDING AREA. SERVICED RESERVOIR & INSTALLED CAP IAW CHECKLIST REV 07-01-2011 AND MM 72-00-00. RAN ENGINE AT DIFFERENT POWER SETTINGS & LEAK CHECKED WITH NO LEAKS NOTED. OPS CHECKED GOOD.