If you own a Honda 400ex, you know it’s a reliable workhorse. But keeping it running strong means using the right lubricants. This guide covers everything you need to know about Honda 400ex oil type, capacity, and more to keep your ATV in top shape.
Using the correct oil is not just a suggestion—it’s critical for engine longevity and performance. The wrong type or amount can lead to overheating, increased wear, and even catastrophic failure. Let’s break down the specifics so you can maintain your quad with confidence.
Honda 400ex Oil Type, Capacity, And More
This section gives you the core specifications for your TRX400EX. We’ll cover the exact oil types Honda recommends, the total capacities for different services, and why these details matter so much.
Recommended Oil Type and Viscosity
Honda specifies a few key options for your 400ex. The manual recommends using a high-quality, four-stroke motorcycle oil.
- Pro-Honda GN4 10W-40: This is Honda’s own petroleum-based oil and is a great, cost-effective choice for most riding conditions.
- Pro-Honda HP4 10W-40: Available in both synthetic blend (silver bottle) and full synthetic (gold bottle). Synthetic oils offer better protection under extreme heat and stress.
- General Rule: You can use any JASO MA or MA2 certified 10W-40 motorcycle oil. This certification ensures it’s safe for wet clutches, which is absolutly essential for the 400ex’s combined engine/transmission system.
Never use automotive oil labeled “Energy Conserving” as it can cause your clutch to slip. Stick with 10W-40 for a wide temperature range, though 20W-50 can be used in very hot climates.
Oil Capacity Specifications
The 400ex has a “dry-sump” oiling system. This means oil is stored in an external tank, not just in the engine crankcase. There are two capacities to know: engine oil change and total system capacity.
- Oil Change Capacity (with filter change): Approximately 2.0 US quarts (1.9 liters).
- Total System Capacity (after complete rebuild/run dry): Approximately 2.4 US quarts (2.3 liters).
- Oil Tank Dipstick Check Level: The dipstick is on the oil tank itself, located near the front left wheel. Check it with the engine warm and the ATV on level ground.
Always start by adding less oil, then check the dipstick and add gradually to avoid overfilling, which is just as bad as underfilling.
Oil Change Intervals
Regular changes are the cheapest insurance for your engine. Follow this schedule:
- Normal Riding: Change oil and filter every 100 operating hours or 6 months, whichever comes first.
- Severe Service (racing, frequent dust, mud, very cold/hot temps): Change oil and filter every 50 operating hours or 3 months.
- Always change the oil filter with every oil change. It’s a small, inexpensive part that traps harmful debris.
Choosing Between Conventional, Synthetic Blend, and Full Synthetic
Your choice depends on your riding style and budget.
- Conventional (like GN4): Perfect for casual trail riding, moderate temperatures, and if you’re sticking to strict change intervals.
- Synthetic Blend: Offers a good balance, providing better heat and shear resistance than conventional oil without the full synthetic price.
- Full Synthetic: The best protection for high-RPM use, racing, or extreme temperatures. It lasts longer and maintains its protective qualities better under stress.
You can switch between types; just try to stick with one for a consistant interval. There’s no need to “flush” the system when switching.
Step-by-Step Oil and Filter Change Guide
Changing the oil yourself is straightforward. Here’s how to do it correctly.
Tools and Materials You’ll Need
- 2 quarts of 10W-40 JASO MA/MA2 motorcycle oil
- Honda 400ex oil filter (Honda part #15412-MN5-003 or equivalent)
- 17mm wrench for drain plugs
- Oil filter wrench (strap or cap style)
- Oil drain pan
- Funnel
- Clean shop rags
- New washer for engine case drain plug (optional but recommended)
The Change Procedure
- Warm Up the Engine: Run the ATV for 5-10 minutes. Warm oil flows out easier, carrying more contaminants with it.
- Drain the Oil Tank: Place the drain pan under the oil tank drain plug (17mm bolt on the bottom of the oil tank). Remove the plug and let it drain completely.
- Drain the Engine Crankcase: Locate the engine case drain plug on the bottom of the engine, just behind the shift lever. Use the 17mm wrench to remove it and let this oil drain into your pan too.
- Remove the Old Oil Filter: The filter is on the right side of the engine, near the front. Use your filter wrench to turn it counter-clockwise. Have a rag ready as some oil will spill out.
- Install the New Filter: Lightly coat the rubber gasket on the new filter with fresh oil. Screw it on by hand until the gasket touches, then tighten it an additional 2/3 to 3/4 of a turn. Do not overtighten.
- Reinstall Drain Plugs: Clean the drain plugs and replace the washer if you have a new one. Tighten them securely, but avoid stripping the threads.
- Add Fresh Oil: Pour approximately 1.8 quarts of new oil into the oil tank fill hole (on top of the oil tank). Use your funnel.
- Check the Level: Start the engine and let it idle for 1-2 minutes. This circulates oil and fills the new filter. Turn off the engine, wait a minute, then check the dipstick. Add oil slowly until the level is between the upper and lower marks. Do not overfill.
- Clean Up and Check for Leaks: Wipe any spilled oil, start the engine again, and look for leaks around the filter and drain plugs. Dispose of the old oil responsibly at a recycling center.
Understanding the Dry-Sump Oil System
The 400ex uses a dry-sump system, which is common on performance machines. Here’s why it matters:
- The oil is stored in a separate tank, not in the engine crankcase.
- A scavenge pump pulls oil from the engine and returns it to the tank.
- A pressure pump then sends oil from the tank back to the engine.
- This design allows for a larger total oil capacity, better cooling (the oil tank acts as a cooler), and prevents oil starvation during hard cornering or wheelies.
This is why you check the oil level in the tank, not on an engine dipstick. Always check it with the engine warm and after it’s been run for a moment to ensure an accurate reading.
Common Oil-Related Problems and Solutions
Even with proper care, issues can arise. Here are a few common ones.
Oil Burning or Consumption
If your 400ex is smoking (blue-ish smoke) or using oil excessively, it’s often due to worn piston rings or valve seals. This requires engine disassembly to fix. Using a slightly heavier oil (like 20W-50) can sometimes reduce consumption in an older engine, but it’s a temporary fix.
Clutch Slipping
If the engine RPMs climb but the ATV doesn’t accelerate proportionally, you may have a slipping clutch. The first thing to check is your oil. Did you accidentally use automotive oil? Drain it immediately and refill with the correct JASO MA oil.
Overheating and Oil Degradation
Riding in deep mud or slow, technical trails can cause overheating. The oil breaks down faster. If the oil smells burnt or becomes very thin, change it immediately. Consider upgrading to a full synthetic and ensure your coolant system is also in good order.
Milky or Frothy Oil
If the oil on the dipstick looks milky white or frothy, that indicates coolant is mixing with the oil. This is a serious problem usually caused by a blown head gasket or a cracked cylinder/head. Do not run the engine. This requires immediate mechanical repair.
Seasonal Oil Considerations
Temperature changes affect your oil’s performance.
- Summer/Hot Weather: Stick with 10W-40 or switch to 20W-50 for sustained high-heat operation. The oil’s resistance to thinning is crucial.
- Winter/Cold Weather: A 10W-40 synthetic will flow better at startup than a conventional 10W-40. In frigid climates, a 5W-40 synthetic motorcycle oil can be used for easier cold starts, but confirm it has JASO MA certification.
- Always allow extra warm-up time in colder weather to let the oil circulate fully before riding hard.
FAQ Section
What kind of oil does a Honda 400ex take?
It takes a 10W-40 four-stroke motorcycle oil that is JASO MA or MA2 certified. Honda recommends their Pro-Honda GN4 or HP4 oils.
How much oil does a TRX400EX hold?
An oil change takes about 2.0 quarts. The entire dry-sump system holds about 2.4 quarts after a rebuild.
Can I use car oil in my Honda 400ex?
No, you should not. Most car oils have friction modifiers that will ruin your wet clutch, causing it to slip. Always use motorcycle-specific oil.
How often should I change my 400ex oil?
Change it every 100 hours or 6 months for normal use. For severe riding, change it every 50 hours or 3 months. Always change the filter too.
Where is the oil filter on a 400ex?
It’s on the right side of the engine, near the front, just above the exhaust header pipe. It’s a cylindrical canister-type filter.
Why is my Honda 400ex leaking oil?
Common leak points include the oil tank drain plug, engine case drain plug, oil filter, or the crankcase seal behind the front sprocket. Check these areas after a cleaning the engine to find the source.
What happens if I overfill the oil?
Overfilling can cause excessive pressure, leading to oil being forced into the airbox or out breather hoses. It can also cause aerated oil, which reduces lubrication and can damage the engine. Always add slowly and check the dipstick frequently.
Maintaining your Honda 400ex with the correct oil is one of the simplest yet most impactful things you can do. By following these guidelines for the right Honda 400ex oil type, capacity, and change procedures, you ensure your quad delivers reliable power for years to come. Remember, clean oil is the lifeblood of your machine—don’t neglect it. Grab the right supplies, set aside an hour, and give your 400ex the care it deserves.