If you own a small engine—be it a trusty lawnmower, a powerful pressure washer, or a reliable generator—you know the key to longevity is proper maintenance. While keeping the fuel fresh is important, the lifeblood of your engine is the motor oil.
Walk into any hardware or auto parts store, and the wall of oil bottles can be overwhelming. The most crucial decision you’ll face is choosing the correct SAE viscosity. Getting this right is the cheapest insurance you can buy for your machine.
Decoding the SAE Viscosity Rating
SAE stands for the Society of Automotive Engineers. The numbers on the bottle, like 5W-30 or 10W-40, are a code that tells you exactly how the oil flows under different temperatures. This is what you need to understand to choose correctly:
1. The “W” Number (Cold Viscosity)
This first number (e.g., the 5 in 5W-30) relates to the oil’s viscosity (thickness) when it’s cold—specifically, how easily it flows at 0°F (-18°C).
- W stands for Winter.
- The lower the number, the thinner the oil when cold.
- Why it matters: On startup, especially in cold weather, oil needs to flow immediately to critical engine parts. A lower ‘W’ number means faster lubrication, which reduces wear and tear that occurs in the first few seconds of running.
2. The Second Number (Hot Viscosity)
The second number (e.g., the 30 in 5W-30) indicates the oil’s viscosity at the engine’s normal operating temperature (usually around 212°F or 100°C).
- The higher the number, the thicker the oil when hot.
- Why it matters: When your engine is hot and running hard (like mowing a thick lawn on a summer day), you need a thick enough oil film to keep metal surfaces separated and prevent friction damage.
Climate Is King: Matching Oil to Temperature
The beauty of multi-grade oils (like 10W-30) is that they use special additives that allow the oil to act like a thin oil when cold (for easy starting) and a thick oil when hot (for engine protection). Your local climate is the primary factor dictating which multi-grade oil you should choose.
| Climate/Season | Recommended Viscosity Type | Why? |
| Very Cold (Below 32°F / 0°C) | 0W-30 or 5W-30 | The low ‘W’ number ensures the oil doesn’t turn to sludge, allowing for quick, damage-free starts. |
| Moderate/All-Season (Year-round use in mild climates) | 10W-30 | The most common choice. It balances cold-start performance with hot-running protection for general use. |
| Very Hot(Consistently above 85°F / 30°C) | 10W-40 or SAE 30 (Straight Weight) | The higher hot number (40) provides a more robust protective film in extreme heat. A straight-weight SAE 30 is common in older small engines during hot summer months, but should never be used when it’s cold. |
Your Engine Manual is Your Bible
While understanding viscosity is crucial, the single most important rule is this: Always follow your engine manufacturer’s primary recommendation.
Engine designers specify a certain range of viscosities based on the tight tolerances and oil passageways within that specific engine.
- Check the Manual First: Find the viscosity rating the manufacturer recommends (e.g., “Use SAE 10W-30”).
- Refer to the Temperature Chart: Most manuals will include a chart that shows a range of acceptable viscosities (e.g., 5W-30, 10W-30, 10W-40) and the ambient temperatures they cover. This is your cue to make adjustments based on your climate.
- Stick to the Approved Range: If you live in a region that sees extreme swings—say, -10°F in winter and 95°F in summer—you may need to switch between the manufacturer’s approved 5W-30 (winter) and 10W-30 or 10W-40 (summer) options. Never use a weight that is not listed in the manual, even if you think it’s “thicker” and offers “more protection.”
The right oil is the best defense against engine wear. Take the time to match the SAE viscosity to your engine’s needs and the demands of your local climate, and your small engine will reward you with years of faithful service.