UPON RETURN FROM ROUTINE FLIGHT; THE OWNER OF A PIPER AEROSTAR 601B NOTIFIES MECHANIC WITH INSPECTION AUTHORITY (IA); HE HAD NOTICED 'RED' FUEL SEEPING FROM IMPROPERLY SEATED MAIN TANK CAP.

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UPON RETURN FROM ROUTINE FLT; THE OWNER NOTIFIED ME THAT HE HAD NOTICED 'RED' FUEL SEEPING FROM IMPROPERLY SEATED MAIN TANK CAP. HE STATED THAT HE HAD NOTIFIED THE FUEL HANDLER OF THE SITUATION AND REVIEWED FUEL TRUCK FUEL CERTIFICATIONS AND LOGS. THEN HE TOOK A FUEL SAMPLE FROM EACH DRAIN PORT AND WAS SATISFIED THAT THE FUEL WAS OK. I DRAINED FUEL FROM EACH PORT AND NOTICED THAT IT WAS CLR AND HAD AN OILY FEEL TO THE FUEL FROM THE AUX TANK. THE ACFT WAS GROUNDED PENDING RESULTS OF THE FUEL SAMPLE. THE 1 GAL SAMPLE WAS SENT TO FUEL TESTING LAB. THE SAMPLE CONFIRMED THAT JET OR DIESEL IS PRESENT IN THE FUEL. ACFT GROUNDING CONTINUES PENDING INSPECTIONS IN ACCORDANCE WITH LYCOMING SB398. ALL APPLICABLE PARTIES HAVE BEEN NOTIFIED AS WELL AS THE AREA FAA WHERE THE FUEL HAD BEEN PURCHASED. COPIES OF THE SAMPLE RESULTS WERE PROVIDED TO ALL PARTIES. WE WERE UNABLE TO DETERMINE THE EXACT RATIO OF CONTAMINATION OR IF ALL TANKS WERE INVOLVED OR JUST THE AUX TANK BECAUSE THE ACFT HAD BEEN REFUELED ON HIS SECOND LEG WITH EXCEPTION TO THE AUX TANK. 160+ GALS WERE LOADED AT THIS FUELING STATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED HE WAS ORIGINALLY INFORMED BY THE TEST LAB THERE WAS JET OR DIESEL PRESENT IN THE AVGAS FUEL SAMPLE HE HAD SENT TO THE TEST FACILITY. SO THEY DRAINED THE ENTIRE FUEL TANK SYSTEM. THIS WAS FOLLOWED BY A COMPRESSION CHECK OF THE TIO-540-U2 LYCOMING ENGINE. HE ALSO DID AN EXTENSIVE COLORED BORESCOPE INSPECTION OF THE ENGINE CYLINDERS; PISTON HEADS FOR PREDETONATION DAMAGE AND THE VALVE EDGES FOR ANY SIGNS OF METAL FATIGUE; INDICATED BY THE VALVE(S) TAKING ON THE SHAPE OF A TULIP; ALSO KNOWN AS THE 'TULIP EFFECT'; THAT COULD BE CAUSED BY FUEL CONTAMINATION WITH LOWER OCTANE DIESEL OR JET FUEL. NO DAMAGE WAS FOUND. REPORTER STATED FURTHER INVESTIGATION TURNED TO A FUEL ADDITIVE KNOWN AS 'PRIST' THAT ALSO ADDS A REDDISH COLOR TO THE FUEL. REPORTER STATED THE TEST LAB LATER BACKED OFF OF THE POSITION THAT THE TEST SAMPLE WAS DIESEL OR JET FUEL. CURRENTLY; THEY ARE TAKING FUEL SAMPLES EVERY 100 HOURS AND BORESCOPING FOR VALVE DAMAGE THAT WOULD BE THE FIRST INDICATION OF AN ACTUAL LOW OCTANE OR FUEL CONTAMINATION.