What was found
CREW REPORTED, MASTER WARNING & LT OIL PRESS WARNING LIGHTS ILLUMINATED. LT OIL PRESS ZERO. LT ENGINE SECURED AN APPROPRIATE CHECKLIST ACCOMPLISHED. ATC NOTIFIED LT ENGINE SHUTDOWN & PRIORITY REQUESTED. AT 3,000 FT, RT ENG PULLED BACK TO 65 PERCENT N2 & OIL PRESS DROPPED TO 50 PSI IN YELLOW ARC. THE RT ENGINE N2 SET BACK TO APPROX 75 PERCENT, & OIL PRESSURE HAD RETURNED TO ABOVE 60 PSI, IN THE GREEN ARC. AN EMERGENCY THEN DECLARED, & AIRCRAFT LANDED WITHOUT FURTHER INCIDENT. MX REPLACED LT ENG RELIEF VALVE HOUSING PN 3108036-01R, 2 CHECK VALVES PN 3119587-01, AND REPLACED OIL FILTER FOR ANALYSIS. OIL PRESS FOR RT ENGINE FOUND WITHIN LIMITS, & DISCREPANCY COULD NOT BE DUPLICATED. A/C RETURNED TO SERVICE.