Viton replacement O-ring for the constant pressure valve (CPV) on BMW S54 Motors
The factory O-ring often shrinks and hardens causing an oil leak. The leak it causes can be misdiagnosed as a rear main, but is usually just this O-ring. BMW does not sell just the O-ring and the material BMW uses degrades quickly. This Viton O-Ring has a wider range of useful temperatures and will have a longer lifespan than the stock one.
Above is a picture of where the CPV is placed on the block. Simply remove the valve, cut the old O-ring away, and install the new one.