Onan OnaMax SAE 15W-40 Engine Oil is factory tested to offer maximum protection. OnaMax Engine Oil is a conventional, non-synthetic oil specially formulated to reduce consumption and engine wear due to high temperature oil oxidation and viscosity increase. All Onan Generators from the factory are shipped with OnaMax 15W-40. Specially formulated to reduce consumption & engine wear due to high temp oil oxidation 32-oz. bottle | Cummins Onan OnaMax SAE 15W-40 Engine Oil, 32 Oz