THE PLT REPORTED THAT HE LANDED ON A GRASS STRIP ADJACENT TO THE PAVED RWY, TO 'SAVE THE (AIRPLANE) TIRES' FROM EXCESSIVE WEAR. HE STATED THAT THE AIRPLANE GROUNDLOOPED DURING THE ROLLOUT WHEN IT ENCOUNTERED A HOLE IN THE TURF. A MECHANIC WHO EXAMINED THE AIRPLANE & ACCIDENT SITE REPORTED THAT THE AIRPLANE APPARENTLY GROUNDLOOPED AS THE AIRPLANE TURNED ONTO THE PAVED RWY DURING THE ROLLOUT. THE MECHANIC STATED THAT HE FOUND THE ROLLOUT AREA TO BE FREE OF HOLES AND FAIRLY SMOOTH, AND THAT THE RWY EDGE WAS ABOUT 2 INCHES HIGHER THAN THE TURF. THE MECHANIC NOTED THAT THE GRASSY AREA HAD SERVED AS A TEMPORARY RWY THE YEAR BEFORE WHEN THE PAVED RWY WAS CLOSED FOR MAINTENANCE. ACCORDING TO THE APPLICABLE AIRPORT/FACILITY DIRECTORY, THE ARPT HAD ONLY A SINGLE PAVED RWY; NO GRASS RWY WAS LISTED. THE PLT DID NOT FILE AN ACC REPORT.