THE HELICOPTER WAS ON A SWATH RUN AT ABOUT 5 FT AGL & WAS APPROACHING HI OBSTRUCTIONS WHEN THE PLT NOTED THAT THE NEEDLES SPLIT, THE ENG RPM INCREASED & THE MAIN ROTOR RPM BEGAN DECREASING. THE PLT TURNED TO AVOID THE OBSTRUCTIONS, BUT WAS UNABLE TO LEVEL THE ACFT BEFORE GROUND IMPACT. IT HIT THE GROUND IN A RIGHT SIDE, TAIL LOW, ATTITUDE & ROTATED TO THE RIGHT AFTER THE MAIN ROTOR MADE GROUND CONTACT. AN INVESTIGATION REVEALED THE HYDROMECHANICAL CLUTCH HAD BEEN OVERHAULED & INSTALLED, 2.5 FLT HRS PRIOR TO THE ACCIDENT. NO UNUSUAL WEAR, DEFECTIVE PARTS NOR IMPROPER ALIGNMENT OF THE CLUTCH WAS FOUND. THE CLUTCH HAD BEEN LUBRICATED WITH AN UPDATED OIL. THE MILITARY HAD UPGRADE MIL-L-2105 LUBRICANT TO MIL-L-2105B & C WITH NEW ADDITIVES TO PREVENT LOCKUP OF ITS LIMITED SLIP DIFFERENTIALS ON NEWER HELICOPTERS. HOWEVER,THE ADDITIVES WERE NOT COMPATIBLE WITH UH-19D HYDROMECHANICAL CLUTCHES WHICH USED A METAL-TO-METAL PRINCIPLE. AFTER THE ACCIDENT, MIL-L-21260 TYPE 1, GRADE 30 OIL WAS MADE A SUBSTITUTE FOR MIL-L-2105A GRADE 80 OIL, WHICH WAS NO LONGER AVAIL