Motor Oil
Viscosity Selector
Answer 5 questions about your vehicle and driving conditions. Get the exact AMSOIL grade your engine needs — no owner's manual hunting required.
- Questions
- 5
- Time
- ~60 sec
- Grades covered
- 0W-16 → 25W-60
- Tool by
- Alan Williams, Dealer #1243776
What type of vehicle are you choosing oil for?
Select the option that best matches your vehicle. This determines the base viscosity range we start from.
What best describes your typical climate?
Winter temperature matters most — the "W" number in viscosity ratings is chosen based on your coldest expected temperature.
How many miles are on your engine?
Higher mileage engines sometimes benefit from slightly thicker oil to compensate for increased internal clearances and worn seals.
How do you primarily use your vehicle?
Severe use — towing, hauling, off-road, frequent short trips — creates more heat and stress, often requiring a thicker operating-temperature viscosity.
What matters most to you?
This helps us choose between AMSOIL product lines that share the same viscosity grade but are optimised for different goals.
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Shop All AMSOIL Products →How to Read Motor Oil Viscosity Numbers
Every motor oil grade has two numbers separated by a W — for example, 5W-30. The W stands for Winter. The first number (5 in this case) measures how well the oil flows at cold temperatures — specifically how easily your engine can crank it at startup in freezing conditions. The lower this number, the better the oil flows in the cold. A 0W oil flows better at -40°F than a 10W oil does.
The second number (30 in 5W-30) measures the oil's resistance to thinning at full operating temperature — typically around 212–250°F inside a running engine. A higher second number means a thicker oil film at heat. This film is what stands between metal engine components and catastrophic wear.
Viscosity Grade Quick Reference
| Grade | Cold flow down to | Operating temp | Best for | AMSOIL option |
|---|---|---|---|---|
| 0W-16 | -40°F / -40°C | Very thin | Toyota/Honda fuel economy engines | AMSOIL OE 0W-16 |
| 0W-20 | -40°F / -40°C | Thin | Most modern cars/trucks | AMSOIL Signature Series 0W-20 |
| 5W-20 | -22°F / -30°C | Thin | Many Ford/GM/Honda engines | AMSOIL Signature Series 5W-20 |
| 5W-30 | -22°F / -30°C | Medium | Most versatile — widest coverage | AMSOIL Signature Series 5W-30 |
| 0W-40 | -40°F / -40°C | Thick | European engines, performance | AMSOIL Signature Series 0W-40 |
| 5W-40 | -22°F / -30°C | Thick | Diesel trucks, performance, towing | AMSOIL Signature Series 5W-40 |
| 10W-30 | -13°F / -25°C | Medium | Older engines, warmer climates | AMSOIL OE 10W-30 |
| 15W-40 | 5°F / -15°C | Very thick | Diesel trucks, heavy duty | AMSOIL Max-Duty 15W-40 |
| 20W-50 | 14°F / -10°C | Very thick | Harley-Davidson V-Twin, high temp | AMSOIL V-Twin 20W-50 |
When to Go Thicker — and When Not To
Many drivers instinctively reach for a thicker oil thinking "more protection." This is only true up to a point. Modern engines with tight tolerances actually need thinner oils to flow quickly to bearings and VVT actuators at startup. In a new engine, using 10W-40 when 5W-30 is specified can starve components of oil for the critical first few seconds after a cold start — causing more wear, not less.
The exceptions: high-mileage engines with worn seals benefit from seal-conditioning additives in AMSOIL High Mileage formulas, and sometimes from moving one grade thicker (5W-30 → 5W-40) to compensate for increased clearances. Towing and hauling applications genuinely benefit from a thicker hot-side viscosity (the second number) to maintain oil film under sustained high heat and load.
AMSOIL vs Conventional Oil — Does Viscosity Grade Matter?
With conventional oil, viscosity grades are more critical to follow exactly — because conventional oils thin out more rapidly at heat and degrade faster over time, staying close to their rated viscosity requires precise matching. AMSOIL synthetic oils, made from Group IV PAO base stocks, maintain their viscosity grade far more consistently across the entire drain interval. An AMSOIL 5W-30 that's been in your engine for 15,000 miles still behaves more like a 5W-30 than a conventional 5W-30 at 3,000 miles does — because the synthetic base oils resist thermal breakdown far better.
This is one of the reasons AMSOIL Signature Series can be rated for up to 25,000 miles — the oil's protective properties hold throughout the drain interval in a way conventional oils simply cannot match.
Viscosity Questions — Answered
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