Oil filter sealing grommet to heat shield 
The oil filter grommet on your opposed twin Onan is critical.  This grommet can save your motor.  The design of the engine cooling requires that this grommet is in place to keep the cooling air from escaping around the filter housing; that tin gap needs to be filled with this part. With this grommet missing it is very common for catastrophic engine failure, we see this very often.  The most common failures are loose or bad valve seats, piston or head damage, and often broken connecting rods.  Chances are if there is a failed Onan engine with an oil filter, the bad cylinder 85% of the time is on the oil filter side and this grommet is missing.