Engine Oil: What It Does, Which Grade to Use, and How Often to Change It

What Does Engine Oil Do?

Engine oil performs five critical functions inside your engine:

1. Lubrication — forms a thin film between metal surfaces (pistons, crankshaft, camshaft, bearings) to prevent metal-on-metal contact.
2. Cooling — carries heat away from components that water cooling can't reach, such as the pistons and cylinder walls.
3. Cleaning — detergent additives suspend soot, carbon deposits, and varnish, carrying them to the oil filter.
4. Sealing — helps the piston rings maintain compression by filling microscopic gaps.
5. Protection against corrosion — inhibitors in the oil coat internal metal surfaces.

Without oil — or with degraded oil — engine components wear rapidly. An engine starved of oil can seize in minutes.

Understanding Oil Viscosity Grades

Engine oil is rated using the SAE viscosity system (e.g., 5W-30, 10W-40). The number before the "W" (winter) indicates cold-start flow; the number after indicates viscosity at operating temperature.

Always use the grade specified in your owner's manual. Using a grade that's too thick can impede cold-start lubrication; too thin and it won't maintain adequate pressure at operating temperature.

Synthetic vs Conventional Oil

• Fully synthetic oil — engineered molecules that perform consistently across a wide temperature range; lasts longer (up to 10,000–15,000 km between changes); best for turbocharged, high-performance, or modern direct-injection engines.
• Semi-synthetic (part-synthetic) — a blend; good middle ground for most everyday NZ cars.
• Mineral (conventional) oil — refined from crude oil; fine for older, simpler engines; typically changed every 5,000–7,500 km.

How Often Should You Change Engine Oil in NZ?

NZ driving conditions — particularly short trips, stop-start city driving, and towing — can accelerate oil degradation. If you predominantly do short trips around Auckland or Wellington, err towards the shorter interval.

How to Check Your Oil Level

1. Park on level ground and wait 5 minutes after switching off the engine.
2. Locate the dipstick (usually a brightly coloured handle near the front of the engine).
3. Pull it out, wipe clean, reinsert fully, then withdraw again.
4. The oil level should sit between the MIN and MAX marks.
5. Check the colour: honey to amber is healthy; black and gritty means it's overdue for a change; milky or frothy indicates coolant contamination (a serious fault — see a mechanic immediately).

What Happens If You Neglect Oil Changes?

• Sludge build-up — degraded oil forms thick deposits that block oil passages. Costly engine damage results.
• Increased wear — additive depletion means less protection; bearing wear accelerates.
• Overheating — dirty oil loses cooling efficiency.
• Engine seizure — in extreme cases, complete engine failure requiring a rebuild or replacement.

NZ Oil Change Costs

Common NZ cars like the Toyota Corolla, Mazda Demio, and Suzuki Swift are inexpensive to service due to widely available oil grades and filters.

When to Book a Mechanic

• Your service light is on, or you've exceeded your change interval.
• Oil level drops frequently — there may be a leak or your engine is burning oil.
• Oil looks black, milky, or has visible particles — immediate inspection needed.
• You hear a ticking or knocking noise from the engine — often low oil pressure.

Further Reading

• Find a mechanic near you
• Oil Filter: What It Does and Why You Should Change It With Every Oil Change
• Oil Pump: How It Works and What Happens When It Fails