The private pilot stated during a telephonic interview that he was hovering in an open field adjacent to the airport, when he began to loose rotor RPM. He landed briefly to investigate why the helicopter was not flying properly. The pilot then decided to hover taxi back to his hangar and was trying to keep the nose of the helicopter pointed into the wind. While hovering with a slight right crab over a freshly plowed field the helicopter settled, the right skid contacted the soft terrain, and rolled onto the right side. The pilot stated that the loss of rotor RPM was the result of the loose engine oil cap that sprayed engine oil on the main rotor system drive belts. The helicopter main rotor blades, tail rotor blades, fuselage, and tail boom were substantially damaged. There was no post-crash fire and the pilot reported no injuries. Weather was reported at the time as winds from 020 degrees at 7 knots, visibility 30 miles with few clouds at 7,000 feet and a temperature of 72 degrees.