The Honda 400EX typically uses a 10W-40 motorcycle oil (API SN or newer), with Honda GN4 10W-40 serving as a common, manufacturer-approved option. The engine and transmission share this oil, so you don’t need separate gear oil for the gearbox. Always confirm the exact specification for your model year in the owner's manual.
Oil type and grade
Choosing the right oil affects lubrication, cooling, and overall performance. For the 400EX, the standard guidance is to run a 10W-40 motorcycle oil that meets API SN or newer. You may choose conventional, synthetic blend, or full synthetic, but automotive engine oils are not recommended for ATVs of this type due to additive packages and viscosity considerations. The oil also serves the transmission in these models, so a single fill is used for both engine and gearbox.
Options to consider include:
- 10W-40 motorcycle oil (API SN or newer); conventional or synthetic blends
- 10W-40 full synthetic motorcycle oil (API SN or newer)
- Honda GN4 10W-40 as a brand-specific option
Bottom line: for typical use, a 10W-40 motorcycle oil meeting API SN or newer is the standard choice.
Capacity and maintenance
Oil capacity and maintenance specifics can vary by model year and whether you replace the oil filter. The engine oil capacity with a new filter is generally in the vicinity of 1.8–2.0 quarts (about 1.7–1.9 liters). After warming the engine, check the level with the bike level and fill to the indicated range; avoid overfilling to prevent foaming or leaks.
Maintenance intervals depend on riding conditions and the manual’s guidance. In normal, casual use, many riders follow a schedule around every 50 hours of operation or at least annually, with more frequent changes in dusty, hot, or severe environments. Always consult your year-specific owner’s manual for the exact interval.
- Engine oil capacity with filter change: approximately 1.8–2.0 quarts (1.7–1.9 liters)
- Recommended change interval: follow the manual; commonly around 50 hours or annually for casual riding; more frequent in harsh conditions
- Check level when the oil is warm and the quad is on a level surface; use the dipstick/level marks per the manual
Using the right oil and sticking to the maintenance schedule helps protect the engine and transmission, especially under demanding riding conditions.
Summary
In short, the Honda 400EX uses a 10W-40 motorcycle oil meeting API SN or newer, with the Honda GN4 10W-40 being a widely recommended option. Expect a capacity around 1.8–2.0 quarts with a filter, and follow the model-year-specific maintenance schedule in the owner’s manual. In hot climates or under heavy use, more frequent changes may be prudent. Always verify the exact specs for your bike’s year and regional guidance.