If you own a motorhome, your generator is one of the most important pieces of equipment onboard. Our Onan QG 5500 generator has been a reliable workhorse, powering the air conditioning on hot days, keeping the fridge running when we’re off-grid, and giving us peace of mind when shore power isn’t available.
But like any engine, it needs regular maintenance. One of the most important things you can do is a routine oil and filter change. It’s simple, inexpensive, and can save you from major repairs down the road.
In this post, I’ll walk you through how I change the oil and filter in our Onan QG 5500.
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Why Regular Maintenance Matters
Cummins Onan recommends changing the oil every 150 hours or once a year — whichever comes first. For us, it usually ends up being a yearly oil change, since we don’t rack up enough generator hours to hit the 150-hour mark.
Skipping oil changes can lead to dirty oil circulating through the engine, causing extra wear, harder starts, and reduced lifespan. Spending 30–40 minutes once a year is cheap insurance for reliable performance.
Equally important: exercise your generator monthly. Run it under load (we usually kick on the A/C) for about 30 minutes. Generators are a bit temperamental if they sit unused, and monthly runs keep the carburetor clean, seals lubricated, and the electrical windings in good shape.
What You’ll Need
Here’s what I use for an Onan QG 5500 oil and air filter change:
- Oil: We use 15W-40, which covers most conditions. Always check the temperatures you’ll be running your generator in to ensure you’re using the correct oil weight for your climate. The owner’s manual includes a chart for this.
- Oil filter: Onan 122-0836 or equivalent (I replace mine at every oil change)
- Air filter: Onan 140-3116 or equivalent
- Oil drain pan
- Funnel Spout
- Rags or paper towels
- Disposable gloves
Step-by-Step Oil Change
1. Warm Up the Generator
Run the generator for about 5 minutes to warm the oil. This helps it drain faster and more completely.
2. Access the Oil Drain
- Remove the front service panel.
- Locate the oil drain valve on the lower right side of the generator.
- Position your oil pan underneath.
3. Drain the Oil
- Remove the oil fill cap — this allows air in so the oil flows out smoothly.
- Turn the drain valve and let the oil flow into the drain pan.
- Once the oil is drained, close the valve.
(No tools are needed — the built-in quick-drain valve makes this super easy.)
4. Replace the Oil Filter
- Use an oil filter wrench to remove the old filter.
- Lightly oil the gasket of the new filter.
- Fill the filter with fresh oil
- Thread it on by hand and snug it down. Don’t overtighten.
5. Add New Oil
- Using a funnel, pour in about 2 quarts of fresh oil through the fill opening.
- Check the dipstick and add small amounts until it’s right at the “Full” mark.
6. Change Air Filter Too
And while you’re already servicing the generator, it’s a good idea to also check and replace the air filter. A clean air filter ensures proper airflow, better combustion, and helps your generator run more efficiently.
7. Run & Recheck
- Start the generator and let it run for a few minutes.
- Shut it down, wait a minute, then recheck the dipstick.
- Top off if needed.
Final Thoughts
For us, changing the oil and filter in our Onan QG 5500 generator is a once-a-year ritual that keeps it reliable for all our travels. I always change the filter at the same time as the oil — it’s a small cost for big peace of mind.
Just as important, we make sure to exercise the generator monthly under load. A little attention goes a long way toward keeping it dependable, especially when you need it most — whether that’s powering the A/C in the Texas heat, keeping batteries charged in the mountains, or running the microwave for a hot meal after a long hike.
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