EMS SMT ACFT FLC HAD NMAC WITH SMA SEL BENEATH AN ARSA AIRSPACE.

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ENRTE FROM XYZ TO ZZZ FOR PATIENT PICKUP. MY ACFT ENCOUNTERED A 1/10 - 2/10 SCATTERED LAYER OF CLOUDS AT ABOUT 1800 MSL (AT 1000 AGL). WHILE CLBING TO CONTINUE ABOVE SCATTERED LAYER; 2500 FT MSL WE EXPERIENCED A MECHANICAL PROBLEM (#1) TRANS OIL PRESSURE WENT TO '0'. WE ABORTED FLT AND WERE DSNDING BACK TO LAND AT ABC. AS WE PASSED BTWN THE CLOUDS AT 18-1900 MSL (COVERAGE ABOUT 1/10 AT THIS POINT) SUDDENLY A FIXED WING ACFT; POSSIBLY AN ACFT WHITE IN COLOR; APPEARED OUT OF THE SMALL CLOUD TO OUR R; ABOUT 300-400 FT AWAY HDG STRAIGHT TOWARD OUR SIDE. I REDUCED PWR AND IMMEDIATELY PLACED THE HELI IN A DIVE PASSING JUST AHEAD AND BELOW THE ONCOMING FIXED WING. PRECAUTIONARY LNDG WAS ACCOMPLISHED AT ABC WITHOUT FURTHER INCIDENT. WHILE ACFT WAS BEING REPAIRED; THE ACFT WAS SEEN AGAIN SLOWLY FLYING AROUND THE ABC AREA GOING IN AND OUT OF CLOUDS. THIS AREA IS JUST OUTSIDE THE ABC'S ARSA NORMAL SVC AREA. I WAS NEVER ABLE TO GET A TAIL NUMBER NOR DETERMINE WHICH DIRECTION THE OTHER ACFT FINALLY WENT. MY DSCNT INTO ABC WAS VFR AT ALL TIMES; (WE HAD THE LNDG ZONE IN SIGHT; WHEN NEAR MISS OCCURRED). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. NO ADDITIONAL COMMENTS DURING EMS STRUCTURED CALLBACK.