Viscosity is simply a measure of how thick or thin a fluid is, specifically its resistance to flow. In engine oil terms, viscosity determines how well the oil can circulate through your engine at different temperatures. Too thick and the oil struggles to flow quickly at startup, leaving components momentarily unprotected. Too thin and it cannot maintain the film strength needed to prevent metal-to-metal contact under load.

Modern engine oils are multigrade, meaning they are formulated to behave appropriately across a wide range of temperatures rather than being optimised for just one condition.

Take a common grade like 5W-30 and it tells you two distinct things:

• The “5W” (Winter rating): The number before the W indicates how the oil behaves in cold temperatures. A lower number means the oil stays fluid in colder conditions, allowing it to circulate quickly at startup. A 5W oil flows more easily in winter than a 10W or 15W oil. This is critical because the majority of engine wear happens in the first few seconds after a cold start, before the oil has fully circulated.
• The “30” (Operating temperature rating): The number after the W indicates the oil’s viscosity at normal operating temperature (100 degrees Celsius). A higher number means the oil is thicker at operating temperature and maintains a stronger film under load and heat. A 40 weight oil offers more protection under high stress than a 30 weight, but may slightly reduce fuel efficiency in everyday driving.

So a 5W-30 oil is thin enough to flow quickly in cold conditions but builds adequate film strength once the engine reaches operating temperature. A 10W-40 is slightly thicker at both ends of the scale and is often used in older or higher-mileage engines where a bit more film thickness helps compensate for worn clearances.

Beyond viscosity, oil is also classified by performance ratings that indicate what types of engines and conditions it is designed for:

• API ratings (American Petroleum Institute): You will see codes like SP, SN, CK-4, or CJ-4 on the bottle. The letter S indicates the oil is for petrol engines, while C indicates diesel. The second letter indicates the performance generation, with later letters in the alphabet representing more recent and more stringent standards. SP is the current top tier for petrol engines. CK-4 is designed for modern diesel engines meeting Euro VI emissions requirements.
• ACEA ratings (European Automobile Manufacturers Association): European manufacturers tend to use ACEA classifications, which are often more stringent than API in certain areas. You will see codes like A3/B4 or C3 on premium oils. C-class ACEA ratings indicate low-SAPS (sulphated ash, phosphorus, and sulphur) formulations required for vehicles with diesel particulate filters and catalytic converters.

The base oil used in a lubricant has a significant impact on its performance and longevity:

• Fully synthetic oils are engineered at a molecular level, providing the most consistent viscosity, the best low-temperature flow, the highest resistance to oxidation and degradation, and the longest service intervals. They are the best choice for modern turbocharged, direct-injection, and hybrid engines running tight tolerances and extended oil change intervals.
• Semi-synthetic (part-synthetic) oils blend synthetic base stocks with refined mineral oil. They offer better performance than straight mineral oil at a more accessible price point, making them a solid choice for everyday vehicles and light commercial use.
• Mineral oils are refined from crude oil and represent the traditional formulation. They are perfectly adequate for older engines, classic vehicles, and machinery with larger clearances that were designed around mineral oil specifications. Using a modern full synthetic in a very old engine can sometimes cause seal issues, making mineral oil the better choice.

Reading your owner’s manual is the definitive answer, but for most people that book is buried in the glovebox or lost entirely. The OilHub rego lookup tool takes all of the guesswork out of the equation. Enter your New Zealand plate number and the system matches your specific vehicle to the correct oil grade, specification, and quantity. No cross-referencing, no guessing, no buying the wrong thing.

OilHub also sells in exact quantities, so you are not stuck buying a 5-litre bottle when your engine only takes 4.2 litres. That means less waste and less money spent on oil that ends up sitting in your garage.