Another episode of WHEN THREADS STRAY...
I have seen two incidents of where grossly overfilling the crackcase of inline 4cyl Onan 12.5 Kw Onans led to failure of piston oil control and compression rings. This possibility needs to be eliminated in a process of troubleshooting. If compression testing reveals acceptable cylinder pressure the spark plugs should be pulled and examined. If the center electrode is black and shint then the oil consumption is internal. A blown rear crankshaft seal may allow the stator body to fill with oil.
Just some possibilities...
