After taxiing a Cessna C-208B aircraft from a tie-down to a cargo loading area; a pilot notices oil leaking near the propeller hub. Maintenance trouble shooting locates oil coming from a high pressure oil tube at the front elbow on their Pratt-Whitney PT6A-114A engine.

Narrative

After taxi; repositioning Cessna C-208B [Aircraft X] from Ramp (tie down area) to ZZZ Ramp (Cargo loading area). After engine shutdown; observe oil leak coming possibly from propeller hub. Maintenance personnel inspected and removed part of the engine cowling next to the Oil Cooler Radiator and requested an engine start and run for a minute to detect the specific location where the oil leak came from. After that event; Maintenance personnel found oil coming from front elbow pressure tube (pressure tube of oil system of PT6A-114A engine).Why the event occurred is unknown or theoretical; but today February 2011; when I look at Aircraft X's flight Logbook; I found that in December 2010; January XA; 2011; January XB 2011 and January XC 2011; three different Pilots in Command (PIC) reported nose wheel shimmy. I suggest that a comparison with a previous event (about one year ago) in another C208B aircraft; to find any similarity with previous reported discrepancies. And in case that any similarities [are] found; I suggest that a Pratt and Whitney [Representative be] consulted about the possible cause and any recommendation they [can] give to avoid a future event like this; because this event could have a critical or fatal outcome if aircraft was airborne when an O-ring of any high or low oil pressure line broke.