About 20 min after takeoff, while cruising at 5000 ft, the plt observed oil on the windshield & noted a decrease in oil pressure. He turned back toward the departure arpt & reduced the power in an attempt to prolong eng life. A short time later, the eng began running rough, then failed. The acft was over an area of gnd fog & the plt could not see the gnd. He began gliding at a min rate of descent & trimmed the acft. Before entering the fog, the occupants fastened their seat belts & braced themselves for impact. Shortly after the acft entered the fog, it hit 6 trees, then crashed in a clearing. During a post-crash exam, the oil dip stick was found behind the eng & there was indications that oil had escaped from the top of the eng. The #1 connecting rod had failed & punctured the eng case. A teardown of the eng revealed evidence that the eng had been operating without oil prior to the rod failure.