How to read engine oil viscosity (5W-30, 10W-40, 0W-20)

Every engine oil bottle carries a code such as 5W-30, 10W-40 or 0W-20. It is the first thing we look at, and often the most misunderstood. This guide explains what the viscosity code tells you, what else you need to check and how to arrive at the right lubricant for your engine.

What viscosity is

Viscosity is a fluid's resistance to flow: put simply, how thick or thin it is. An engine oil has to do two things that seem to contradict each other. It must flow quickly to every part of the engine at a cold start and, at the same time, keep a stable lubricating film when the engine is hot and under load.

Modern oils are multigrade: they behave like a thinner oil in the cold and like a thicker oil at operating temperature. That is why the code has two numbers.

Reading the SAE code

The viscosity scale for engine oils is defined by SAE (Society of Automotive Engineers).

• The first number with the letter W (Winter) refers to low-temperature behaviour. The lower it is, the more easily the oil flows at a cold start. A 0W oil flows more easily in the cold than a 10W or a 15W.
• The second number refers to viscosity at engine operating temperature. A 40 is thicker at operating temperature than a 30, and a 30 thicker than a 20.

So a 5W-30 and a 10W-30 behave similarly when the engine is hot, but the 5W-30 circulates faster at a cold start.

Viscosity is not the only criterion

Two oils with the same code, for example two 5W-30 oils, can be completely different products. The difference lies in the specifications they meet:

• ACEA: European categories. A/B categories cover petrol and diesel passenger car engines, while C categories cover oils compatible with exhaust after-treatment systems such as diesel particulate filters (DPF) and catalysts.
• API: American categories, for example API SN or API SP for petrol engines.
• Manufacturer approvals: codes such as VW 504.00/507.00, MB 229.51, BMW Longlife-04 or GM dexos. They show that the oil meets that manufacturer's requirements.

Your engine needs an oil with both the right viscosity and the right specifications. This matters especially in modern diesels with a DPF, where an oil not intended for such systems can affect how they work.

Where to start: the service book

The most reliable source is always the vehicle's owner's manual or service schedule. There you will find:

1. the viscosity or viscosities the manufacturer allows, sometimes depending on climate,
2. the specification or approval required,
3. the oil and filter change interval.

With these details in hand, compare them with the specifications listed on each product. If your engine calls for a specific approval, choose an oil that states it explicitly.

Examples from the EVO range

The PARNAS range of passenger car motor oils covers different requirements. A few examples based on the specifications each product lists:

• If the manual calls for VW 504.00/507.00 or MB 229.51, see EVO SYNTHESIS II SAE 5W-30.
• For engines requiring ACEA C3 with approvals such as MB 229.31/229.51 or BMW Longlife-04, there are EVO SYNTHESIS SAE 5W-30 and EVO SYNTHESIS SAE 5W-40.
• For engines requiring API SP, ILSAC GF-6A or GM dexos1 Gen 2 in 5W-20, there is EVO SYNTECH SAE 5W-20.
• For low viscosities such as 0W-20 and 0W-30, see EVO SYNTHESIS SAE 0W-20 and EVO SYNTECH SAE 0W-30.
• Where the manufacturer allows thicker oils such as 15W-40 or 20W-50, there is EVO SUPER.

The match is always made on your engine's specifications, not on the viscosity code alone.

Frequently asked questions

Can I use a thicker oil for "more protection"? Only if the manufacturer allows it. Modern engines are designed around specific viscosities, and a different oil can affect cold-start lubrication or systems such as variable valve timing.

Topping up with a different oil? It is best to top up with the same oil that is already in the engine. If that is not possible, choose an oil with the same viscosity and the same specifications.

How often should the oil be changed? According to the manufacturer's service schedule. Under severe conditions, such as many short trips, city traffic or towing, many service schedules call for shorter intervals.

How often should I check the level? Regularly, using the dipstick with the engine off on level ground, following the instructions in the manual.

Need help?

If you are not sure which oil suits your engine, send us the make, model, engine and the specification listed in the service book, and we will suggest the right product. See all our products or contact us.