FLC OF SMT JET MADE A 180 DEG TURN; DUE TO WX AND 1 ENG FAILURE; PRIOR TO DECLARING AN EMER.

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WE WERE RETURNING THE ACFT TO HOME BASE AFTER A MAJOR SCHEDULED INSPECTION OF THE ACFT. WE WERE CRUISING AT 41000 FT ABOUT TO PENETRATE A LONG LINE OF LEVEL 3-5 TSTMS. WHEN WE WERE ABOUT 15 NM N OF THE LINE; WE NOTICED OIL PRESSURE ON THE L ENG WAS BEGINNING TO DECLINE. THIS SIT CONTINUED FOR A COUPLE OF MI. THE CHKLIST INSTRUCTED US TO REDUCE PWR ON THE ENG. WHEN WE DID THIS; WE GOT A WARNING LIGHT INDICATING INSUFFICIENT OIL PRESSURE TO CONTINUE RUNNING THE ENG. WE IMMEDIATELY CONTACTED ATC TO TELL THEM OF OUR PROB; AND ANNOUNCED THAT WE WANTED TO TURN AWAY FROM THE LINE OF STORMS WHICH WAS NOW ONLY SEVERAL MI AWAY. I DON'T RECALL IF THE CTLR AUTH THE TURN BECAUSE WE WERE BUSY SECURING THE ENG AND TRYING TO AVOID THE WX. WE MADE A 180 DEG TURN AND BEGAN A DSCNT BECAUSE WE COULD NOT HOLD 41000 FT ON ONE ENG. WE HAD PREVIOUSLY BEEN CLRED TO DSND TO 35000 FT; PD BEFORE WE HAD THE ENG PROB. ALTHOUGH THE CTLR WAS AWARE OF OUR PROB; WE WERE SO BUSY THAT I'M NOT SURE IF WE REMEMBERED TO DECLARE AN EMER. WE THEN CHKED WX AND DECIDED TO LAND AT MEM. WE MADE A NORMAL SINGLE ENG APCH AND LNDG. THERE WERE NO OTHER INFLT PROBS AND THE CTLRS WERE VERY HELPFUL ON THE WAY TO MEM AND UPON ARR. THE FIRE AND RESCUE TRUCKS WERE ON THE RWY WAITING FOR US; SO I BELIEVE THAT ATC TREATED THIS AS AN EMER. UPON LNDG; WE DISCOVERED THAT ALMOST ALL OF THE OIL HAD BEEN BLOWN OUT OF THE L ENG DUE TO A FAULTY 'O'-RING THAT WAS INSTALLED DURING THE INSPECTION. THE ACFT HAS SINCE BEEN REPAIRED WITH NO EVIDENCE OF PERMANENT DAMAGE TO THE ENG. THE PURPOSE FOR THIS RPT IS THAT; DUE TO THE ENG PROB COMBINED WITH THE CLOSE PROX OF LEVEL 5 TSTMS; WE MADE AN IMMEDIATE DECISION TO TURN THE ACFT 180 DEGS. WE ANNOUNCED THE TURN; BUT I'M NOT SURE THE CTLR GRASPED THE FACT THAT WE WERE TURNING COMPLETELY AROUND UNTIL THE TURN WAS COMPLETE. THUS; HE MAY NOT HAVE UNDERSTOOD THAT WE HAD AN EMER UNTIL AFTER THE TURN WAS COMPLETE. ON RECONSIDERATION; I BELIEVE THAT WE SHOULD HAVE IMMEDIATELY DECLARED AN EMER. BUT; CONCERN ABOUT THE LOSS OF THE ENG COMBINED WITH THE SEVERE WX KEPT US BUSY ENOUGH THAT WE DIDN'T IMMEDIATELY MAKE A FORMAL DECLARATION OF THE EMER.