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Firman P03603 / P03608 Oil Type, Capacity and Maintenance Schedule

Updated 2026-11-02 · Written and checked by the Manual Matters team against the manufacturer documents listed under Sources.

Short answer: the FIRMAN P03603 and P03608 (3650 running / 4550 starting watts, 208cc) take 20.3 oz (0.6 L) of engine oil. FIRMAN's recommended type is 10W-30 automotive oil, with a temperature chart for 5W-30, 10W-40 and full-synthetic 5W-30. Fill to the H mark on the dipstick and do not overfill. Change the oil after the first 25 hours on a new engine, then every 100 hours (the schedule says 100 hours or annual). Check the level before each use or at least every 8 hours of operation. All figures come from FIRMAN's owner's manuals for these two models, downloaded from FIRMAN's own product pages.

The P03603 and P03608 manuals give the same engine figures: oil capacity, schedule, spark plug and break-in. If you own a different FIRMAN model, use the manual for that model number. FIRMAN's engines and tank sizes vary across the range.

Key specs (P03603 / P03608 manuals)

Which oil: 10W-30, or something else?

The manual's note says the recommended oil type is 10W-30 automotive oil, but that outdoor temperature determines the right viscosity. Its chart shows:

• 10W-30 across the normal range of outdoor temperatures.
• 5W-30 for the colder end of the chart.
• 10W-40 for the hotter end.
• Full synthetic 5W-30 across the whole chart, from the coldest to the hottest temperatures shown.

For most owners using the generator in typical weather, the 10W-30 that FIRMAN recommends is the answer. If you store and start it in hard winter cold, the chart permits 5W-30 or full-synthetic 5W-30. Look at the chart in your copy of the manual for the exact temperature bands.

FIRMAN is strict about the first fill: do not crank or start the engine until it has been filled with the recommended type and amount of oil. The manual says damage from skipping this voids the warranty.

How to add and check oil

From the "Add engine oil" section:

1. Put the generator on a flat, level surface.
2. Clean around the fill and remove the yellow oil fill cap/dipstick. Wipe the dipstick clean.
3. Using a funnel, pour oil slowly into the fill opening up to the "H" mark on the dipstick.
4. Do not overfill. FIRMAN warns that overfilling can make the engine not start or start hard.
5. Refit the cap/dipstick and tighten it fully.
6. Check the level before each use or at least every 8 hours of operation.

Low oil shutdown: the two traps

The engine has a low oil shut-off and stops when the oil falls below the threshold. The manual flags two situations:

• It shuts off but the oil looks fine: check whether the generator is sitting at an angle that makes the oil shift. Move it to an even surface.
• It will not start at all: the oil level may not be high enough to deactivate the low oil switch. Make sure the sump is completely full, to the H mark.

If the level is below the LOW mark, add oil to the HIGH mark and restart. If the engine stops again, FIRMAN says a low oil condition may still exist: do not keep restarting it, and contact an authorized FIRMAN service dealer. For general no-start causes, see our generator won't start after storage checklist.

Break-in: the first 5 hours

FIRMAN treats the first 5 hours of run time as the break-in period. During it:

• Stay at or below 50% of the running watt rating. On a 3650 running-watt P03603 or P03608, that is about 1825 W.
• Vary the load occasionally so the stator windings heat and cool. FIRMAN says the changing engine speed also helps seat the piston rings.
• Do not apply a heavy electrical load during this period.

Then the first oil change comes at 25 hours, not at 5 hours. Compare the Champion 3500 (5-hour break-in and a 5-hour first change) and the Predator 3500 (about 30 hours). These rules really do differ between brands.

Maintenance schedule

From the P03603 manual's maintenance chart:

FIRMAN adds that you should change the oil more often in extremely dirty or dusty conditions or extremely hot weather. Once a year, it recommends cleaning or replacing the spark plug and replacing the air filter.

Changing the oil

1. Disconnect and ground the spark plug wire. FIRMAN says to do this before any service to prevent accidental starting.
2. With the engine warm, remove the drain plug and the oil filler cap and drain the oil into a container.
3. Reinstall the drain plug.
4. Refill until the oil reaches the HIGH (H) level, about 20.3 oz (0.6 L).
5. Avoid prolonged skin contact with used oil and wash exposed skin with soap and water, as FIRMAN advises. Recycle the old oil.

Air filter and spark plug

• Air filter (foam): take out the element, clean it well in kerosene and dry it. Wet it with clean engine oil, squeeze it tight by hand, then put it back in the case and install it securely.
• Spark plug: remove the cap, remove the plug with the supplied wrench, clean it with a wire brush, and set the gap to 0.7–0.8 mm (0.028–0.031 in). Thread it in by hand first to avoid cross-threading. To measure a gap, see our spark plug gap guide.

Fuel and storage

• Use clean, fresh unleaded gasoline, minimum 87 octane, with no more than 10% ethanol. Above 5,000 ft, FIRMAN allows 85 octane minimum.
• Used infrequently: run it at least 30 minutes every month to avoid hard starting.
• Stored more than a month: the manual's procedure is to add fuel stabilizer, run 5–10 minutes, remove the gasoline from the tank, run it until it stops from lack of fuel, change the oil, and put about a teaspoon of oil through the spark plug hole.
• FIRMAN also says not to keep gasoline from one season to the next.

Draining versus stabilizing is compared in Run it dry or stabilize it?. Before you run it during an outage, read generator placement and extension cord sizing.

More guides: Portable Generators.

FAQ

How much oil does a Firman P03603 take?

20.3 oz (0.6 L). Fill slowly to the H mark on the dipstick with the generator level, and do not overfill.

What oil does a Firman generator use?

FIRMAN recommends 10W-30 automotive oil for the P03603 and P03608. The manual's temperature chart also allows 5W-30 for cold weather, 10W-40 for hot weather, and full-synthetic 5W-30 across all temperatures.

When is the first oil change on a Firman P03603?

After the first 25 hours on a new engine. After that, every 100 hours or once a year, and more often in dusty conditions or very hot weather.

What is the spark plug gap on a Firman P03603?

0.7–0.8 mm (0.028–0.031 in). The manual lists Torch F6RTC, NGK BPR6ES or Champion RN9YC.

Why won't my Firman start after an oil change?

Check that the oil is at the H mark. FIRMAN says a level too low may not release the low oil switch, and overfilling can also cause hard starting or no start.

Sources

• FIRMAN P03603 operator's manual (specifications, add engine oil and temperature chart, low oil shutdown, break-in, maintenance schedule, oil change, spark plug, storage), via FIRMAN's P03603 product page: https://cdn.shopify.com/s/files/1/0527/3350/9822/files/P03603_Operators_Manual.pdf
• FIRMAN P03608 owner's manual, English (same oil capacity, schedule, spark plug and break-in), via FIRMAN's P03608 product page: https://cdn.shopify.com/s/files/1/0527/3350/9822/files/P03608_Manual_English.pdf
• FIRMAN product pages: https://firmanpowerequipment.com/products/p03603 and https://firmanpowerequipment.com/products/p03608