Entrained gas bubble behavior in an engine oil was investigated using a fired 21 DOHC passenger car engine. The gas bubble content in an oil increases proportionally with the engine rotation speed. The engine output and oil temperature also affect the entrained gas bubble content but are not dominant factors. Some negative effects of entrained gas in an engine oil on a running engine were investigated with attention focused on main gallery oil pressure and hydraulic lifter oil pressure. The authors propose some vortex flow type gas-oil separators which are capable of being installed in an engine oil line.