FLC OF AN LTT RETURNED TO LAND AFTER 1 ENG OIL PRESSURE WAS BELOW ACCEPTABLE LIMITS AND ALL PRECAUTIONARY CHKLIST PROCS WOULD NOT BRING IT BACK. HOWEVER; SINCE THERE WERE NO OTHER INDICATIONS OF AN ACTUAL ENG MALFUNCTION THE ENG WAS LEFT RUNNING.

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I WAS THE PF WHILE THE CAPT WAS WRITING DOWN INFO FOR AN ENG MONITORING TREND. (THIS ENG TREND IS DONE FOR OUR MAINT DEPT ONCE A DAY IN VMC BTWN 20000-10000 FT MSL.) ZAU TOLD US TO DSND FROM 11000 FT TO 9000 FT MSL. JUST THEN I NOTICED THAT THE R OIL PRESSURE GAUGE WAS READING ABOUT 30 PSI. (NORMAL OPERATING RANGE IS 90-120 PSI AND CAUTION IS 60-90 PSI.) I THEN REDUCED THE PWR ON THE R ENG TO IDLE AND BROUGHT BOTH PROPS TO MAX RPM. AFTER A SHORT DISCUSSION; THE CAPT AND I DECIDED NOT TO SHUT THE ENG DOWN. THERE WAS NO SECONDARY INDICATION OF LOW OIL PRESSURE. THE R OIL PRESSURE LOW LIGHT WAS NOT ON. (WE TESTED THE LIGHT AND IT WAS WORKING NORMALLY.) THE R OIL TEMP WAS IN THE GREEN AND ABOUT 5 DEGS C LOWER THAN THE L; THE TORQUE AND OIL PRESSURE GAUGES WERE NOT FLUCTUATING (BOTH MEASURE OIL PRESSURE); AND THERE WAS NOT VISUAL INDICATION OF A LEAK. THE EMER CHKLIST CALLS FOR (IN THE EVENT OF A LOW OIL PRESSURE INDICATION) EITHER A PRECAUTIONARY SHUTDOWN OR MINIMUM PWR TO SUSTAIN FLT. BECAUSE THERE WAS NO SECONDARY INDICATION AND BECAUSE MILWAUKEE WAS IFR (300 FT CEILING AND 1 1/4 MI VISIBILITY) WE ELECTED TO NOT SHUT DOWN THE ENG. THE CAPT INFORMED ZAU OF THE PROB. WE DECIDED NOT TO DECLARE AN EMER. HOWEVER; WE DECIDED TO RETURN TO MKE. ABOUT 15 NM SE OF MKE THE CAPT BECAME PF. THE CAPT ELECTED TO HAVE THE CFR STANDING BY. AFTER INTERCEPTING FINAL FOR THE ILS RWY 7R AT MKE THE CAPT ELECTED TO FEATHER THE R ENG (AND KEEP IT RUNNING). WE LANDED WITHOUT INCIDENT. AFTER INVESTIGATING THE PROB; OUR MECHS FOUND THAT THE OIL LEVEL WAS NORMAL AND THERE WERE NO BROKEN LINES. AN ENG RUNUP WAS PERFORMED. IT WAS THEN DETERMINED THAT THE PROBABLE CAUSE WAS EITHER A BAD GAUGE OR A BAD SENDER UNIT. THE ACFT WAS SIGNED OFF WITH A NEW GAUGE AND SENDER. NO FURTHER PROBS OCCURRED.