A BEECH 55 AT 8000 FT DIVERTS DUE TO PARTIAL LOSS OF PWR ON THE L ENG BUT MAKES NO LOGBOOK ENTRY ON ENG PERFORMANCE.

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I AM 1 OF 2 OWNERS OF A 1978 BARON E55 XXXX. ON 04/SAT/02; I DEPARTED ZZZ ON AN IFR FLT. AFTER I LEVELED OFF AT 8000 FT; MY LOW THRUST INDICATOR LIGHT AND HORN CAME ON FOR THE L ENG; AND I WAS CONCERNED THAT I WAS LOSING PWR (ONLY PARTIALLY) IN THE L ENG. I THINK I WAS TALKING TO ZZZ CTR AT THE TIME. I WAS IMC AND TURB WAS MODERATE. WITH THE THREAT OF LOSING PWR IN THE L ENG; I TOLD CTR THAT I WAS EXPERIENCING PARTIAL LOSS OF PWR IN THE L ENG AND REQUESTED VECTORS TO AN ARPT CLOSE BY. THE CTLR VECTORED ME TO ZZZ1. I DID NOT DECLARE AN EMER. AFTER DSNDING TO ABOUT 5000 FT; THE LOW THRUST INDICATOR LIGHT AND HORN THAT HAD BEEN ON TO THAT POINT STOPPED (THE LIGHT WENT OFF AND THE HORN SILENCED). SIMULTANEOUSLY; IT FELT AS THOUGH I HAD FULL PWR BACK IN THE L ENG. NONETHELESS; I LANDED AT ZZZ1 AND WAS SURPRISED TO BE MET BY AN EMER VEHICLE (ARPT FIRE TRUCK). SINCE IT WAS SATURDAY; THE LCL MAINT SHOP WAS CLOSED; BUT I CALLED THE OWNER AND DESCRIBED WHAT I HAD EXPERIENCED. HE RECOMMENDED THAT I PULL THE PROP THROUGH ON THE L ENG AND ASCERTAIN IF 1 OR MORE OF THE 6 CYLINDERS FELT DIFFERENT. I DID THAT AND FELT NOTHING WAS AMISS. HE ALSO RECOMMENDED THAT I RESTART AND RUN UP THE ENGS. WHEN I TRIED TO RESTART THE BATTERIES WERE STILL WEAK (I HAD AN APU START AT ZZZ AND I DID NOT HAVE ENOUGH BATTERY PWR TO RESTART THE ENGS. SINCE IT WAS GETTING LATE IN THE AFTERNOON AND THE WX WAS QUESTIONABLE; I CALLED MY LAW FIRM'S FULL-TIME PROFESSIONAL PLT AND HAD HIM FLY FROM ZZZ2 AND GET ME. I LEFT THE BARON AT ZZZ1 TO BE CHKED OUT AND TO HAVE NEW BATTERIES INSTALLED. ON 04/THU/02; I DIRECTED OUR LAW FIRM PLT TO TAKE SOMEONE QUALIFIED TO FLY THE BARON; MAKE SURE IT WAS PROPER AND LEGAL TO FLY; AND TO FLY THE BARON HOME. THEY FLEW TO ZZZ1 IN THE MORNING. WX WAS SEVERELY VFR. FIRM PLT AND MR X WHO FLEW THE BARON BACK TO ZZZ2 STARTED BOTH ENGS AND RAN THEM UP. THEY SHUT THE ENGS DOWN AND TALKED WITH THE A&P WHO OPENED THAT A 'SLUG OF OIL' HAD MOMENTARILY CLOGGED THE SPARK PLUGS CAUSING A MOMENTARY PARTIAL LOS OF PWR. THE A&P ASSURED BOTH PLTS THAT THE PLANE WAS AIRWORTHY AND THAT THERE WAS NOTHING ELSE TO DO. THE A&P MADE NO LOGBOOK ENTRY OTHER THAN FOR THE 2 NEW BATTERIES; AND THE BARON WAS RESTARTED AND TOOK OFF. THE BARON XXXX FLEW HOME FLAWLESSLY. ON APR/FRI/02; I RECEIVED A PHONE CALL FROM A ZZZ2 FAA REPRESENTATIVE STATING THAT I VIOLATED AN FAR BY ALLOWING THE BARON TO DEPART ZZZ WITHOUT HAVING A PROPER LOGBOOK ENTRY MADE BY THE A&P. WHEN I CALLED THE OWNER OF THE SHOP; HE TOLD ME THERE WAS NOTHING FOR THE A&P TO ENTER IN THE LOGBOOK BECAUSE NOTHING WAS WRONG WITH THE ENG.