SAE viscosity explained
5W-30 or 5W-40: how to compare engine-oil viscosity
Learn what 5W-30 and 5W-40 describe, what viscosity cannot prove and which manual, specification and document checks come next.
Reviewed 2026-08-28
Short answer
5W-30 or 5W-40: how to compare engine-oil viscosity
Both oils share the 5W cold-temperature grade but have different SAE grades at operating temperature. That difference does not make one universally better, and it does not confirm ACEA, API, OEM approval or vehicle fitment.
Read both parts of the SAE grade
The number before W relates to low-temperature performance in the SAE classification. The number after the hyphen identifies the high-temperature viscosity grade. A 5W-30 and a 5W-40 therefore share one part of the classification, not the whole technical profile.
SAE grades are not a quality ranking. A higher second number is not automatically an upgrade and should not replace the grade permitted by the manufacturer.
Check what the viscosity label does not say
The grade does not state the ACEA or API category, a vehicle-maker requirement, the formulation limits for an emissions system or the approved service interval. Two 5W-30 products can be intended for different requirements.
Use the exact TDS and product record to check the rest of the specification. Similar packaging or a shared viscosity is not evidence that two oils are interchangeable.
- manual-permitted SAE grade;
- ACEA or API category;
- OEM specification or approval;
- DPF, GPF and service-interval conditions.
Choose in the order set by the manufacturer
Start with the manual and any temperature chart it provides, then match the full performance and OEM requirements. Do not change grade solely because of climate, mileage or a general internet recommendation.
Where more than one grade is allowed, the operating conditions and the manufacturer instructions decide between them; the viscosity label alone cannot.
Frequently asked questions
Frequently asked questions
They have different high-temperature SAE grades and the same 5W winter grade. That observation does not decide which one your engine requires.
Can I replace 5W-30 with 5W-40?Only when the manufacturer permits the grade and the required ACEA, API and OEM conditions also match. Do not decide from viscosity alone.
Does 5W-30 automatically mean an oil is suitable for DPF?No. DPF-related requirements depend on the performance category, formulation and OEM instructions, not simply the SAE grade.
Limitations
Limitations
- This comparison explains the SAE label; it is not an oil recommendation for a specific engine.
- Operating temperature, engine condition or mileage must not be used to override the manufacturer documentation.
Sources
Verified product records
Related guides
Related guides
Lubrix Oils
Need an exact requirement checked?
Send the engine or equipment details, the requirement from the manual and the exact product name. Availability and commercial terms are confirmed in the reply.
Contact Lubrix Oils