THE NON-INSTRUMENT RATED PLT WAS ON FERRY FLT FROM BARTLESVILLE, OK TO ONTARIO, CA. AN EN ROUTE STOP WAS MADE AT TUCUMCARI, NM, WHERE THE ACFT WAS REFUELED & 14 QTS OF OIL WAS ADDED TO THE LEFT ENG. THE LINE BOY WHO SERVICED THE ACFT SAW NO VISIBLE OIL LEAKS OR SPILLAGE AROUND THE ENG. WHILE ON THE NEXT LEG OF THE FLT, THE ACFT CRASHED NEAR INTERSTATE HIGHWAY 40. TWO WITNESSES THAT SAW THE CRASH STATED RAIN WAS FALLING, THE WIND WAS BLOWING VERY STRONG & THERE WERE LOW CEILINGS. REPORTEDLY, A SQUALL LINE WAS MOVING THRU THE AREA & THERE WAS MODERATE TO SEVERE TURBULENCE. AN EXAM OF THE WRECKAGE REVEALED THE ACFT HAD CRASHED IN A STEEP NOSE DOWN ATTITUDE, WITH 20 TO 30 DEG LEFT BANK & AT HIGH SPEED. NO PREIMPACT, MECHANICAL FAILURES WERE FOUND & BOTH ENGS HAD EVIDENCE THAT THEY WERE OPERATING DURING IMPACT. THE WX BRIEFER SAID THE PLT INDICATED HE WOULD FLY AROUND THE AREA OF PRECIPITATION & REMAIN VFR. RADAR INFO SHOWED THE PLT HAD BEEN FOLLOWING THE INTERSTATE HIGHWAY & VARYING HIS ALT BETWEEN 7900 & 6300 FT MSL.