PLT OF AN SMA SEL ACFT EXPERIENCED ERRATIC INDICATIONS OF THE ACFT XPONDER ALTIMETER; ALTIMETER; AIRSPD AND VERT SPD INDICATORS DURING LEVEL FLT CRUISE.

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I WAS CRUISING AT 5000 FT. THE AIR WAS SMOOTH AND THE ATTITUDE OF THE AIRPLANE NEVER MOVED FROM LEVEL. OCCASIONALLY THE ALTIMETER WOULD JUMP ABOUT 100 FT ABRUPTLY; THEN COME BACK TO 5000 FT. THE AIRSPD INDICATOR AND VSI WOULD ALSO JUMP. ZHU RPTED THAT MY ENCODER WAS SHOWING 4700 FT. I REPLIED THAT I WAS LEVEL 5000 FT. I KNEW I HAD A STATIC SYS PROB; BUT THIS AIRPLANE WASN'T EQUIPPED WITH AN ALTERNATE STATIC SOURCE. IT WAS VFR OUT AND SINCE THIS WAS A RENTAL AIRPLANE; I DIDN'T WANT TO BREAK THE VSI. THE ALTIMETER STARTED GOING CRAZY AND WENT ALL THE WAY DOWN TO 4400 FT; THEN BACK UP TO 4800 FT; WHERE IT STABILIZED. WHEN I GOT ON THE GND AT JAN; I DISCOVERED THE SOURCE OF THE STATIC PROB. IT HAD BEEN A VERY LONG DAY. BEFORE THE FLT; I HELPED THE ACFT OWNER (A&P AND CFI) CHANGE THE OIL IN THE ACFT. I HELPED PUT THE COWL ON AND THEN WENT INSIDE WHILE HE RAN UP THE ACFT. I HAD PREFLTED THE ACFT EARLIER AND IT WAS OTHERWISE READY TO GO. MY PAX AND I JUMPED IN THE ACFT AFTER THE OWNER GAVE US THE THUMBS UP. I DIDN'T DOUBLE- CHK TO VERIFY THAT THE OIL CAP WAS PUT BACK ON; AND IT WASN'T. OIL HAD STREAKED OUT OF THE COWLING AND DOWN THE SIDE OF THE ACFT; RIGHT OVER THE STATIC PORT. I CLEANED UP THE MESS; ADDED OIL AND FLEW HOME. THE ENCODER WAS 100-200 FT LOW WHEN I ASKED JAN APCH TO GIVE ME THE ALT SHOWING ON THE WAY BACK TO NEW. WHEN THE ALTIMETER STARTED JUMPING AROUND; I DID RPT IT TO ZHU. RELEARNED AN OLD LESSON. NEVER TRUST ANYONE; ALWAYS CHK. THE SAYING ABOUT PEOPLE WHO ASSUME RINGS TRUE.