Can you use 10W40 instead of 10W 30?
In most cases, you should follow the vehicle manufacturer’s specification. A 10W-40 oil can be acceptable only if the manual explicitly allows it, and the decision depends on climate, engine design, and service conditions.
Understanding what the numbers mean
The two numbers in a multigrade oil describe how the oil behaves at different temperatures. The “W” stands for winter and indicates cold-start performance; the first number (before the W) shows cold-viscosity characteristics, while the second number (after the W) indicates viscosity at engine operating temperature. A 10W-30 and a 10W-40 both share the same cold-start rating (10W), so their behavior on starting a cold engine is similar. However, at normal engine temperatures, 10W-40 remains thicker than 10W-30, providing a stronger lubricating film but potentially increasing drag and reducing fuel economy. Vehicle makers specify a single viscosity or a range based on engine design, emissions controls, and expected operating conditions. Always check the owner’s manual and the oil specification codes (API, ILSAC, ACEA) before switching.
When 10W-40 might be acceptable
There are conditions where a manufacturer may allow or even recommend a thicker oil. The list below outlines typical scenarios where 10W-40 could be considered, provided the manual or service bulletin confirms it.
- Hot climates or high ambient temperatures, where a thicker oil helps maintain lubrication film strength at sustained high engine temps.
- Older or high-mileage engines with wear that results in looser tolerances or reduced oil-clearance performance, where a thicker oil can help maintain oil pressure.
- Vehicles used for heavy towing, hauls, or frequent high-load driving in warm conditions, which stress lubrication systems.
- Explicit manufacturer guidance that 10W-40 is an acceptable alternative to 10W-30 in the owner’s manual or official bulletins.
- Oil specifications that list 10W-30 or 10W-40 as acceptable options for the same engine family or service interval.
In each case, the manual is the decisive authority. If the manufacturer does not approve 10W-40 for your engine, using it could affect lubrication, emissions controls, and warranty coverage.
When you should not use 10W-40
There are clear drawbacks to switching to a thicker oil unless the maker approves it. The list below highlights situations where 10W-40 is usually not recommended.
- Very cold climates where engine starting and warm-up require maximum flow of oil; thicker oil can delay lubrication on startup.
- Engines that specify a low-viscosity oil (for example, 0W-20, 5W-30, or 0W-16) to meet fuel-economy and emissions targets; using 10W-40 may violate the specification.
- Warranties or service bulletins that require a specific viscosity; using an unapproved grade can void coverage.
- Modern turbocharged or direct-injected engines where precise oil flow is crucial for lubricating turbo bearings and spray cooling; a thicker oil can impede lubrication and cooling in some designs.
- Concerns about fuel economy or increased engine drag due to higher oil viscosity.
Always revert to the manufacturer’s guidance if there is any doubt, and consult a technician if you notice unusual oil consumption, noises, or oil-pressure readings after a switch.
Practical steps if you’re considering switching or have already switched
If you’re weighing the switch to 10W-40 or have recently used it, use the following steps to minimize risk and protect the engine:
- Check the owner’s manual or official service bulletins to confirm whether 10W-40 is an approved alternative to 10W-30 for your exact engine and model year.
- Ensure the 10W-40 oil you select meets the same specification (API/ILSAC/ACEA) as the 10W-30 you were using; cross-check viscosity grade on the oil label.
- Ask your technician to verify any potential impact on warranty, emissions controls, and oil-pressure expectations before making the switch.
- Monitor engine performance after changing: listen for unusual noises, watch for changes in oil pressure readings, and note any changes in oil consumption or fuel economy.
- At the next scheduled oil change, revert to the recommended viscosity if the manual does not authorize continued use of 10W-40; otherwise, continue only if the manual supports it and there are no adverse effects.
Taking these precautions helps ensure that a viscosity switch does not compromise engine protection or warranty coverage.
Summary
Choosing between 10W-30 and 10W-40 should be driven by the vehicle’s official specifications. While 10W-40 can offer a stronger lubricating film in hot climates, high-load use, or with worn engines, it is not universally suitable. Always consult the owner’s manual or a trusted technician before making a switch. If a manufacturer explicitly allows 10W-40 as an alternative, ensure the oil meets the same API/ILSAC/ACEA specs and monitor engine behavior after the change. Revert to the recommended viscosity at the next oil change if advised, and stay attentive to oil pressure, consumption, and any unusual noises.
