When you’re standing in front of a wall of oil bottles at your local Repco Or Supercheap Auto, it can feel like trying to decipher a secret code. You see numbers and letters like 5W-30, 10W-40, or 15W-50, but what do they really mean? Understanding this “oil code” is crucial for keeping your vehicle running smoothly.
Let’s break it down so the next time you’re shopping for oil, you’ll know exactly what you’re looking at.
The Basics: What’s in a Number?
The numbers and letters on oil bottles refer to the oil’s viscosity, which is a fancy way of saying how thick or thin the oil is and how it flows at different temperatures. Viscosity is crucial because it affects how well the oil can lubricate your engine’s moving parts.
- The First Number (e.g., 5W, 10W, 15W)The first number, followed by a “W,” represents the oil’s viscosity in cold temperatures. “W” stands for “Winter,” so this number tells you how easily the oil will flow when your engine is cold. In colder regions, like the South Island during winter, you’d want oil with a lower first number, such as 5W, to ensure the oil can flow easily when you start your engine on a frosty morning.
- The Second Number (e.g., 30, 40, 50)The second number indicates the oil’s viscosity at the engine’s operating temperature (when it’s fully warmed up). The higher this number, the thicker the oil will be at high temperatures. For example, in a place like Auckland, where the temperatures don’t fluctuate as dramatically, a 5W-30 oil would be a good all-rounder.
Top Tip: Follow your manufacturer’s specifications; a thinner oil may be required for some cars due to tighter tolerances.
Why It Matters
We have a diverse climate—from the icy winters in the South to the milder temperatures in the North—which means choosing the right oil is key. Using oil that’s too thick in a cold environment can make your engine struggle to start, while oil that’s too thin in hot conditions may not provide adequate protection.
Here’s a quick guide based on typical New Zealand conditions:
- Colder Climates (South Island winters, high altitudes): Consider oils like 5W-30 or 10W-30. These provide easier flow during cold starts and good protection when the engine is warm.
- Warmer Climates (North Island, coastal regions): 10W-40 or 15W-40 oils work well, offering better protection as temperatures rise.
Synthetic vs. Mineral Oil
- Full-Synthetic oils: These are chemically engineered to perform better in extreme temperatures, making them a great choice for those who drive in varied New Zealand conditions. They offer better protection and longer-lasting performance but come at a higher cost. Typically, they last between 12,000 and 15,000 km, depending on driving conditions.
- Semi-synthetic oils: A mixture of synthetic and mineral oils, these are a good in-between option. Semi-synthetic oils are usually offered at a lower price point than full-synthetic and mineral oils. Typically, they contain around 30% synthetic oil, though this can vary. They last longer than mineral oils but may not offer the same extended intervals as full-synthetic oils, typically requiring a change every 7,500 to 10,000 km.
- Mineral oils: Derived directly from crude oil, these are usually cheaper. They work well for standard driving conditions and regular vehicle maintenance schedules. However, they require more frequent changes, typically every 5,000 to 7,500 km, depending on driving conditions and engine type.
Top Tip: Don’t skimp on oils. This is the only thing that keeps your engine lubricated. If you can, opt for a good quality oil and change it regularly!
Picking the Right Oil:
Need help choosing the right oil? Talk to your local mechanic or check your vehicle’s owner’s manual for manufacturer recommendations. Staying informed is the first step to maintaining a healthy engine.
Top Tip: Using Repco or Supercheap Auto’s part-by-plate system can make it easy to identify which oil will fit your car.
