A LEARJET 36 EXPERIENCED LOW OIL PRESSURE AT CRUISE ALT AND ELECTED TO MAKE A PRECAUTIONARY LNDG. DIPSTICK SEAL LEAKING.

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MY COPLT AND I LEVELED OUR ACFT OFF AT FL330 FOR THE FIRST LEG OF OUR TRIP. ABOUT 15 MINS INTO CRUISE; WE NOTICED THAT THE OIL PRESSURE ON OUR #1 ENG APPEARED TO BE LOWER THAN NORMAL. WE TOLD ONE ANOTHER THAT WE'D KEEP AN EYE ON IT AND IF IT BECAME A PROB THAT WE WOULD DIVERT. ABOUT 5 MINS PASSED AND WE THEN NOTICED THAT THE OIL PRESSURE HAD FALLEN INTO THE CAUTION ZONE FOLLOWED BY THE RED ZONE. WE RECEIVED THE MASTER WARNING LIGHT THAT COMES ON WHEN THE OIL PRESSURE REACHES 25 PSI FOLLOWED BY THE LOW OIL AND OIL PRESSURE WARNING LIGHTS ON THE ANNUNCIATOR PANEL. THE COPLT WAS THE PF SO I TOLD HIM TO CONTINUE FLYING THE ACFT WHILE I OPENED THE CHKLIST. I THEN PROCEEDED TO PULL THE #1 ENG TO IDLE SO AS NOT TO PUT UNNECESSARY STRAIN ON IT AND TO POSSIBLY SAVE SOME OF THE OIL AS IT WAS NOT DETRIMENTAL TO OUR CURRENT FLT CONDITIONS. I WENT THROUGH THE CHKLIST WHICH STATED TO MONITOR THE ENG INSTS (IE; THE OIL TEMP AND OIL PRESSURE GAUGES); BUT IF THE OIL PRESSURE WENT BELOW 25 PSI THAT WE WOULD HAVE TO SHUT THE ENG DOWN. AT THAT POINT; ZLA HAD GIVEN US A DSCNT TO FL290 WHICH WE SO DID. THE FO AND I AGREED THAT THE BEST COURSE OF ACTION AT THIS TIME WAS TO DIVERT TO PALM SPRINGS WHICH WAS ABOUT 30 NM TO OUR SW. I THEN PROCEEDED TO TELL CTR THAT WE NEEDED TO DIVERT TO PALM SPRINGS FOR A PRECAUTIONARY LNDG. I DID NOT DECLARE AN EMER AT THIS TIME BECAUSE WE STILL HAD ENG PWR AVAILABLE. AT THIS POINT THE OIL PRESSURE APPEARED TO STABILIZE AT 25 PSI AND WAS NOT GOING LOWER AS WITH THE OIL TEMP REMAINING IN THE GREEN ARC; SO WE OPTED TO KEEP THE ENG RUNNING AS LONG AS IT REMAINED WITHIN PARAMETERS. WE CONTINUED OUR DSCNT AND RECEIVED A VISUAL APCH TO RWY 31L AT PALM SPRINGS. BY THE TIME WE WERE LINED UP ON FINAL APCH; I NOTICED THE OIL PRESSURE TO BE GOING DOWN TO 20 PSI; WE OPTED TO KEEP THE ENG RUNNING UNTIL LNDG; WHICH WE DID. WE PERFORMED A FLAPS 20 DEG LNDG; ROLLED OUT; SHUT DOWN THE #1 ENG AND TAXIED TO THE RAMP WITHOUT FURTHER INCIDENT. AFTER WE PARKED THE ACFT; WE WENT OUTSIDE FOR A VISUAL INSPECTION. WE FOUND OIL ALL ALONG THE COWLING AND TAIL; THE DIPSTICK WAS DRY; AND THERE WAS OIL POURING FROM THE BOTTOM OF THE COWLING. I CONTACTED MY MAINT DEPT; CHARTER DEPT; AND SPOKE WITH THE DIRECTOR OF OPS TO ADVISE THEM OF THE SIT. I SQUAWKED THE DISCREPANCY; SECURED THE ACFT; AND REFUSED TO FLY IT UNTIL AN INSPECTION WAS PERFORMED. AS IT TURNS OUT; THERE WAS A BAD O-RING ON THE OIL DIPSTICK CAP FOR THE #1 ENG. PERHAPS THERE SHOULD BE A MORE FREQUENT INSPECTION REQUIRED TO CHK THINGS LIKE THIS ON LEARJETS.