Summary
The main difference between 5W-30 and 5W-40 oil appears at operating temperature: 5W-40 belongs to a higher viscosity class when hot. Both have the same 5W rating for their winter performance. However, this does not mean they are interchangeable, as ACEA, API standards, and manufacturer-specific approvals are as important as viscosity.
What do 5W, 30, and 40 mean?
The number before the W classifies the oil's behavior at low temperatures. Since both oils are rated 5W, they belong to the same SAE winter class. The number 30 or 40 then describes a kinematic viscosity range at 100 °C: an SAE 40 is more viscous than an SAE 30 under these conditions.
These numbers do not measure the overall quality or lifespan of the oil. Modern oil is a combination of a base and additives designed to meet specific tests.
Why is manufacturer approval a priority?
Two 5W-30 oils can have different ash content, shear resistance, and pollution control performance. The vehicle manual indicates a standard or approval to be followed. This reference takes into account the engine, turbocharger, oil change intervals, and, where applicable, the particulate filter.
Therefore, do not automatically replace a 5W-30 with a 5W-40 for a high-mileage engine. Higher viscosity is not a repair for oil consumption, a leak, or mechanical wear.
When is 5W-30 encountered?
5W-30 is common in many modern engines. Depending on its formulation, it can help reduce friction and be suitable for certain after-treatment devices. But not all 5W-30s are "low SAPS," and not all have the same approval.
The presence of a DPF, a specific catalyst, or extended service intervals requires reading the full label, not just the front of the bottle.
When is 5W-40 encountered?
5W-40 maintains a higher viscosity when hot and is specified for many gasoline or diesel engines. It may be intended for severe conditions or designs requiring this class, but it is not universally better.
If the manufacturer allows several grades, ambient temperature and usage can guide the choice. In the absence of such an option in the manual, stick to the prescribed specification.
Can you top up with the other viscosity?
In an emergency, restoring a correct level is often preferable to driving with insufficient oil. Use the product as close as possible to the required viscosity and approval, without exceeding the maximum, then ask if an early oil change is necessary.
Before any modification or maintenance around the engine, also check the exact compatibility of the parts presented in our Audi engine, intake, and exhaust collection.