The flight was uneventful during takeoff and climb to an assigned cruise altitude of FL260. Approximately 40 minutes after takeoff the number 3 engine experienced an uncontained failure of the turbine blades on the number 4 power turbine wheel. Although debris from the engine penetrated the pressure vessel, the pilot made an unscheduled landing at Fresno Airport, California without further incident. Examination of the engine revealed that ferrous chips were contaminating, and oil was bypassing the engine oil filter for at least 40 days. However, no corrective action was taken by company maintenance personnel. The oil and filter were changed three times in the three months previous to the accident. The examination revealed that the contaminated oil contributed to the failure of the number four turbine bearing subsequently leading to overheating and damage to the overspeed sensors. The turbine oversped and the turbine blades departed the turbine disc. Probable cause: Failure of company maintenance personnel to adequately examine the engine lubrication system when contamination was detected.