AN EMB120 FLC DIVERTS TO AN ALTERNATE WHEN THE L ENG INDICATES IT IS LOW ON OIL.

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DURING CRUISE ON A FLT FROM LFT TO DFW; WE RECEIVED A WARNING OF LOW OIL PRESSURE IN THE L ENG. FOLLOWED COMPANY CHKLIST PROCS AND CALLED COMPANY. WE DIVERTED TO TXK AND DECLARED EMER WITH CTR. UPON ARR AT TXK; WE WERE SEQUENCED #2 BEHIND ANOTHER ACFT ON APCH TO RWY 31. WE INFORMED CTR THAT WE NEEDED TO LAND ON RWY 4; AS IT WAS LONGER AND STRAIGHT IN. WE WERE TOLD TO EXECUTE A L 360 DEG TURN. WE QUESTIONED THIS WITH CTR; BUT HE SAID IT WAS NECESSARY FOR SEPARATION. WE RELUCTANTLY COMPLIED SINCE WE DID NOT HAVE A NEED TO LAND IMMEDIATELY. I FELT THAT THIS WAS NOT ACCEPTABLE; AS WE WERE AN EMER. THE CTLR SHOULD HAVE INSTRUCTED TXK TWR TO MAKE THE OTHER ACFT GO AROUND. IN TXK; MAINT INSPECTED ENG; AND ADDED 4 QUARTS OF OIL. WE RAN UP THE ENG AND AGAIN INSPECTED IT. NO LEAKS; AND MAINT CTL SHOWED NO HIGH CONSUMPTION HISTORY. THE RUN-UP SHOWED NOMINAL OIL PRESSURE. IT WAS NOTED THAT THE OIL SIGHT GAUGE WAS VERY DARK; AND IT LOOKED AS IF IT WERE FULL OF OIL WHEN IT WASN'T. MAINT IS RESPONSIBLE FOR CHKING OIL LEVELS; ALTHOUGH FLCS DO FOR SAFETY'S SAKE. MY FO SAID HE HAD CHKED THE OIL LEVELS; AND HE THOUGHT THEY WERE FULL. THE DIRTY/DARK SIGHT GAUGE HAD FOOLED HIM; AND PROBABLY MAINT AS WELL. THE SIGHT GAUGES SHOULD BE KEPT CLEAN OR REPLACED IF THEY BECOME DIFFICULT TO READ.