Best Engine Oil Viscosity: Boost MPG Without Trying
Best engine oil viscosity for fuel efficiency
The best engine oil viscosity for fuel efficiency is usually the lowest viscosity your vehicle manufacturer approves, most often something like 0W-20, 5W-20, or 0W-30. Lower-viscosity oils reduce internal friction, which can improve fuel economy, but the right choice still depends on your owner's manual, engine design, climate, and warranty requirements.
In plain terms, thinner oil flows faster through the engine, especially during cold starts, and that can cut drag and help the engine waste less fuel overcoming its own resistance. But using oil that is thinner than specified can reduce protection, so "best for fuel efficiency" does not mean "thinnest possible".
Why viscosity matters
Oil viscosity describes how easily oil flows at low and high temperatures, and the SAE grade on the bottle tells you how that oil behaves in cold starts and normal operating heat. A lower winter rating, such as 0W instead of 5W, helps oil circulate sooner after startup, when engine wear and friction are often highest.
Fuel efficiency improves when the oil places less mechanical drag on moving parts, and several lubricant makers note that lower-viscosity grades can reduce friction and support better mileage. Research also shows the benefit depends on the engine, route, load, and operating temperature, which is why a low-viscosity oil that works well in one car may not be ideal in another.
Best grades by situation
The most fuel-efficient oil is the one that is both manufacturer-approved and on the low-viscosity end of the approved range for your engine. In many modern gasoline vehicles, that means 0W-16, 0W-20, 5W-20, or 0W-30, while some newer designs are calibrated specifically for these lighter grades.
| Viscosity grade | Fuel-efficiency tendency | Typical fit | Tradeoff |
|---|---|---|---|
| 0W-16 | Very high | Some newer engines designed for ultra-low viscosity | Not suitable unless explicitly approved |
| 0W-20 | High | Many modern fuel-economy-focused gasoline engines | Less margin than thicker oils in severe heat or load |
| 5W-20 | High | Common in many mainstream cars | Slightly slower cold flow than 0W grades |
| 0W-30 | Moderately high | Vehicles needing a little more high-temperature protection | Often a small efficiency tradeoff versus 0W-20 |
| 5W-30 | Moderate | Very common all-purpose grade | Usually a bit less fuel-efficient than thinner options |
| 10W-30 and thicker | Lower | Older engines, hotter climates, or heavier-duty use | More pumping and friction losses, typically worse mileage |
What the evidence says
Industry guidance consistently points in the same direction: lower-viscosity oils generally improve fuel economy because they reduce resistance inside the engine. Castrol notes that lower-viscosity oils improve fuel economy, while Mobil says switching from a higher viscosity oil to a recommended lower viscosity such as 0W-20 or 0W-30 can help support fuel economy, with actual savings depending on vehicle and conditions.
Peer-reviewed research also supports the trend, though the exact savings vary. A 2019 study on low-viscosity engine oils found that they can reduce fuel consumption, but the effect depends strongly on engine type, driving cycle, and friction modifiers, especially under real-world conditions. That means the headline is not "thinner is always better," but rather "the right low-viscosity oil can be measurably better for efficiency when matched to the engine".
"Lower viscosity motor oils also result in better fuel economy."
How to choose correctly
The safest process is simple: first follow the owner's manual, then choose the lowest approved viscosity that fits your climate and driving style. If your manual allows multiple grades, the lower number is usually the more fuel-efficient option, especially for stop-and-go driving and frequent cold starts.
- Check the owner's manual for approved SAE grades and OEM specifications.
- Prefer the lowest approved viscosity for your engine if fuel economy is the priority.
- Make sure the oil meets the required performance standard, not just the right thickness.
- Use the grade that matches your climate and driving severity, especially in hot weather or towing.
- Recheck the recommendation if your vehicle is under warranty or has special engine requirements.
When thicker oil makes sense
Thicker oil can be the better choice when the engine is older, has higher mileage, burns oil, or operates under sustained heat and load. In those cases, protecting the engine may matter more than squeezing out a small fuel-saving gain, because an overly thin oil can create too little film strength between moving parts.
This is why a 5W-30 or 10W-30 may be appropriate in some vehicles even though a 0W-20 might save a little more fuel in a different engine. The "best" viscosity is therefore the one that balances friction reduction with adequate lubrication for your exact engine design.
Common myths
One common myth is that switching to the thinnest oil you can buy always increases mileage; in reality, the gain only appears when the oil is approved for that engine. Another myth is that fuel economy is driven only by oil thickness, when in fact additive chemistry, HTHS viscosity, temperature, and driving conditions also matter.
A third myth is that a thicker oil always protects better in every situation. Protection depends on the engine's clearances, operating temperature, and the manufacturer's engineering targets, so the wrong thick oil can increase drag without delivering any real benefit.
Practical recommendation
If your goal is maximum fuel efficiency, start with the lightest approved oil in your manual, usually 0W-20, 5W-20, or 0W-30 for many modern gasoline cars. If your manual lists only one grade, that is the correct choice, even if another grade sounds "better" for economy.
For most drivers, the biggest fuel-economy mistake is not choosing between 0W-20 and 5W-30 at random, but ignoring the manufacturer's specification altogether. The right viscosity gives you the best mix of efficiency, wear protection, and warranty safety.
Everything you need to know about Best Engine Oil Viscosity For Fuel Efficiency
Is 0W-20 better than 5W-30 for fuel economy?
Usually yes, because 0W-20 is thinner and typically creates less friction, but only if your vehicle manufacturer approves it for your engine.
Can I use thinner oil to save gas?
Only within the grades listed in your owner's manual, because using oil that is thinner than approved can reduce protection and may hurt the engine over time.
Does synthetic oil improve fuel economy?
Synthetic oil can help support fuel economy when it is formulated at a lower viscosity and meets the correct spec, but the viscosity grade matters more than the marketing label by itself.
What is the most fuel-efficient oil today?
For many passenger cars, the most fuel-efficient choice is a manufacturer-approved 0W-16 or 0W-20, but the correct answer is always the lowest approved viscosity for your specific engine.
Should I change viscosity for winter?
Only if the manual allows multiple grades, because a lower winter rating can improve cold-start flow, but the safest choice is still the viscosity range specified by the manufacturer.