On May 29, 2021, about 1015 mountain daylight time, an experimental, amateur-built Glasair II S FT airplane, N8176J, was substantially damaged when it was involved in an accident near Darby, Montana. The pilot and the passenger sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, during the flight, while at an altitude of about 6,800 ft, the engine monitor alerted him to a low oil pressure indication. The oil pressure continued to drop, and the pilot began a descent toward a nearby private grass strip for a precautionary landing. During the final approach, the pilot determined that the airplane was high and fast, and he attempted a slip to slow the airplane and descend quickly. About halfway down the runway, the pilot elected to go around and added engine power before he started a turn. About three quarters of the way through the turn, the pilot “felt the airplane begin to stall.” To avoid powerlines, the pilot pushed the nose down and the airplane contacted the ground in a left-wing-low, nose-down attitude. The airplane was substantially damaged and both occupants were transported to the hospital with serious injuries. Postaccident examination of the experimental, Eggenfellner Subaru engine revealed that the engine crankshaft would not rotate until the dampener off the fly wheel was removed. The crankshaft then rotated through several complete cycles. No binding or abnormal noise was noted, and compression was developed in each cylinder. All six sparkplugs displayed normal operating signatures. About 2 quarts of engine oil was drained out of the engine. The oil sump was removed with no abnormalities noted. The oil filter and oil filter adapter were removed and RTV sealant was noted around the base of the oil filter adapter. The O-ring was nicked and deteriorated. The open oil filter had about an inch long sliver of RTV sealant around the base of the oil filter adapter. Due to engine design and lack of specialized tooling, no further examination or testing were accomplished.