The factory-fill oil shipped in every new Onan generator. Conventional SAE 15W-40, specially formulated to reduce consumption and engine wear caused by high-temperature oil oxidation and viscosity increase. For year-round use in Cummins liquid-cooled and air-cooled RV generator engines. 1-quart bottle.
OnaMax 15W-40 is not just an approved oil for Onan generators — it is the oil Cummins Onan fills every new generator with before it leaves the factory. When it comes time for your first oil change or any scheduled maintenance interval, using OnaMax keeps your engine on the same formulation it was designed and broken in with.
The key performance focus of OnaMax is protection against high-temperature oil oxidation and viscosity increase — the two primary failure modes for generator oil under load. RV generators frequently operate at sustained high loads (full AC draw, appliances, charging) in confined compartments where ambient temperatures run high. Under these conditions, standard automotive oils can oxidize and thicken over time, reducing flow and leaving engine surfaces under-protected. OnaMax's conventional formula is specifically engineered to resist these conditions and maintain consistent viscosity and film strength across extended run times.
The SAE 15W-40 multi-viscosity rating provides reliable cold-start protection in cooler temperatures while maintaining a stable film at full operating temperature — making it a practical year-round choice for both diesel liquid-cooled and gasoline air-cooled Onan generator engines.
Shipped in every new Onan generator from the factory — the OEM-specified oil for Cummins liquid-cooled and air-cooled RV generator engines
Specially formulated to resist oil oxidation and viscosity increase under sustained high-load, high-heat generator operation — maintains film strength where it counts
Engineered to minimize oil consumption under demanding conditions — helps extend service intervals and reduces the need for top-offs between changes
Multi-viscosity rating provides reliable cold-start protection and stable high-temperature film — suitable for year-round use in most climates
Standard automotive oils are formulated for vehicle engines that cycle through warm-up, load, and cool-down. RV generators operate differently — often running continuously at sustained high load for hours at a time, in tight compartments with limited airflow. OnaMax is specifically engineered for these conditions: ✓ Resists oxidation at sustained high temperatures ✓ Maintains viscosity under extended high-load operation ✓ Reduces consumption to minimize top-offs between changes ✓ Protects against wear in both diesel and gasoline engines Refer to your Cummins Onan Operator's Manual for the recommended oil change interval and capacity for your specific generator model.
• One (1) 32 oz bottle of OnaMax SAE 15W-40 Generator Oil Note: Oil capacity varies by generator model. Most Onan RV generator engines require 1.5–2 quarts per oil change. Refer to your Operator's Manual for the correct capacity and change interval for your specific model before ordering quantity.
Backed by a 1-year limited manufacturer's warranty. For best generator performance and warranty protection, Cummins Onan recommends using OnaMax — the Green Label Parts oil specifically formulated and factory-tested for Onan RV generator engines.
Always refer to your Cummins Onan Operator's Manual for the recommended oil type, viscosity grade, capacity, and change interval for your specific generator model. Using the factory-specified oil helps maintain warranty coverage and ensures the generator performs as designed.