A C172 AT 8100 FT RETURNED TO THE FIELD DUE TO LOW OIL PRESSURE AND LOSS OF ENG PWR. CAUSE UNKNOWN.

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STARTUP; TAXI; AND RUNUP WERE NORMAL WITH ALL GAUGES IN THE GREEN. THE ACFT WAS SLUGGISH ON TKOF ROLL AND VERY SLOW TO GENERATE A POSITIVE RATE OF CLB. ALL GAUGES WERE STILL IN THE GREEN BUT ONLY 2500 FPM OBTAINED. THIS WAS MY FIRST FLT IN THIS ACFT. NOT BEING FAMILIAR WITH THIS AIRFRAME; AND KNOWING THAT IT WAS HEAVILY USED IN THE LCL FLT SCHOOL; I ASSUMED THAT IT WAS MERELY 'TIRED.' I INTERCEPTED V81 BUT WAS UNABLE TO GAIN ALT. I ENTERED A HOLDING PATTERN JUST OUTSIDE THE DEN CLASS B AIRSPACE WHILE TRYING TO DETERMINE THE CAUSE OF INADEQUATE PWR. WITH 15 DEGS OF NOSE UP; I WAS ABLE TO ESTABLISH A 100 FPM CLB AT 55-60 KTS. LEVEL FLT RESULTED IN A 300 FPM DSCNT. AT THIS POINT; I OBSERVED THE OIL PRESSURE HAD DROPPED BELOW THE GREEN LINE; OIL TEMP HAD REDLINED; AND RPM HAD DROPPED TO 2300 RPM. BY STARTING A SHALLOW DSCNT; I WAS ABLE TO MAINTAIN 2500 RPM AND BRING THE OIL TEMP SLIGHTLY INTO THE GREEN. SINCE THE ENG APPEARED TO BE STABLE OTHERWISE; I ELECTED TO RETURN TO APA (THE NEAREST ARPT ANYWAY). I ADVISED TWR OF THE SIT BUT DID NOT REQUEST EXPEDITED HANDLING. AN UNEVENTFUL LNDG WAS MADE AND WE WERE ABLE TO TAXI BACK TO THE RAMP. OIL PRESSURE AND TEMP READINGS REMAINED THE SAME UNTIL I THROTTLED BACK FOR SHUTDOWN; AT WHICH TIME IT DROPPED TO APPROX 10 LBS. WHILE THIS INCIDENT ENDED OK; THERE WERE WARNING SIGNS WHICH I FAILED TO INTERP CORRECTLY. DURING PREFLT; I OBSERVED SEVERAL DROPS OF OIL ON THE TARMAC APPROX UNDER THE BREATHER TUBE. THE ACFT HAD JUST BEEN FLOWN AND SHOULDN'T HAVE HAD ENOUGH TIME FOR THIS ACCUMULATION. THE OIL SUMP WAS FULL AND THE OIL COLOR VERY CLEAN EVEN THOUGH THE ACFT WAS ONLY 10 HRS AWAY FROM OIL CHANGE. THIS SHOULD HAVE INDICATED THAT A LOT OF OIL WAS GOING SOMEWHERE AND THAT PREVIOUS PLTS WERE ADDING A LOT OF OIL CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS CHKED BY THE CONTRACTED MAINT PEOPLE AND RPTED NOTHING WAS FOUND. THE RPTR SAID HE CONTACTED AN INDEPENDENT MECH WHO STATED IT MAY HAVE BEEN A RELIEF VALVE STUCK OPEN CAUSING THE LOW PWR. THE RPTR SAID THE FLYING CLUB CONTRACTS THE MAINT AND IT IS BEING DONE IN AN UNSATISFACTORY MANNER.