Walk into any auto parts store and you're faced with a wall of oil. 5W-30, 10W-40, 0W-20, full synthetic, semi-synthetic, mineral, with a dozen different brand claims on every bottle. It's genuinely confusing if you don't know what you're looking for.
Getting this right matters. Using the wrong oil won't always blow your engine on the first drive, but over time it causes wear, sludge, and problems that are expensive to fix.
Here's how to make sense of it.
The Viscosity Number: What It Actually Means
The number on the bottle, like 5W-30 or 10W-40, describes the oil's thickness at different temperatures.
The first number followed by "W" is the winter (cold) rating. Lower means thinner when cold, which means faster flow to engine parts on a cold start when most wear occurs. A 0W or 5W oil reaches critical engine surfaces significantly faster than a 20W on a cold morning.
The second number is the hot viscosity rating. Higher means thicker at operating temperature, which maintains a stronger oil film between moving parts. A 40-grade oil holds its thickness better under heat and load than a 30-grade.
So 5W-30 is thin when cold (good for cold starts) but moderately thick when hot. 10W-40 is slightly thicker cold and thicker hot. Neither is universally better. The right choice depends entirely on what your engine's clearances and tolerances were designed around.
Most modern petrol engines from the last 15 years call for 5W-30 or 0W-20. Many European diesels want 5W-40. Older engines with looser tolerances often prefer 10W-40 or even 20W-50.
Synthetic vs Mineral vs Semi-Synthetic
Mineral oil is refined from crude oil. It works, it's cheap, and it was the standard for decades.
Synthetic oil is engineered at a molecular level to have consistent properties. It flows better when cold, resists breakdown better when hot, lasts longer before degrading, and provides better protection overall. Most manufacturers now require it.
Semi-synthetic is a blend of both. It's a middle ground in both performance and price.
If your car's manual specifies synthetic, use synthetic. Mixing in some mineral oil won't destroy anything immediately, but you're diluting the protection the manufacturer designed the engine around.
For older cars where the manual just says "10W-40" without specifying synthetic: the choice is yours. A 1988 Mercedes-Benz 300SEL with the M103 engine was engineered before full synthetics were common. Running a quality semi-synthetic or mineral 10W-40 is perfectly fine. If anything, some older engines with worn seals actually do better on mineral or semi-synthetic because synthetics can seep through worn gaskets that mineral oil wouldn't.
API and ACEA Ratings: Do They Matter?
API (American Petroleum Institute) and ACEA (European Automobile Manufacturers' Association) ratings certify that an oil meets certain performance standards.
API ratings use letter codes: SN, SP, SL for petrol engines; CK-4, CJ-4 for diesel. Higher letters in the sequence mean newer, more demanding standards. An API SP oil meets a more recent standard than an API SN oil, and generally you can use a higher-rated oil in an engine that required a lower one.
ACEA ratings are common on European and Australian market vehicles. An ACEA A3/B4 rating is common for higher-performance European petrol and diesel engines. ACEA C3 is for modern diesel engines with DPF (diesel particulate filters) and requires low-ash formulations.
Practically: match what your car's manual says. If it says ACEA A3/B4, buy oil that says ACEA A3/B4 on the bottle. Cheaper oils may pass API but not ACEA. Using the wrong spec on a modern diesel with a DPF can damage the filter.
How to Find the Right Spec for Your Car
Check the owner's manual first. It's usually in the maintenance section and will list the exact viscosity and spec required. This is always the most reliable source.
Look for the sticker under the bonnet. Many manufacturers put a summary label near the oil filler cap or in the engine bay. It often lists the oil spec.
Check the oil filler cap itself. Some caps are stamped with the required viscosity.
Use an online oil lookup tool. Most oil brands (Castrol, Penrite, Mobil) have "what oil does my car need" finders on their websites. Enter the year, make, and model, and they'll list compatible products.
If you've got an older or unusual car and the manual has been lost: a forums search for your specific engine code will almost always turn up a thread where someone's already sorted this out.
Older Cars: A Different Set of Rules
Cars from the 1980s and early 1990s have some specific considerations.
Older petrol engines without catalytic converters can use oils with higher zinc (ZDDP) content. ZDDP is an anti-wear additive that was reduced in modern oils because it poisons catalytic converters. For engines with flat-tappet camshafts, which includes most pre-1990s designs, higher ZDDP is actually better protection. Look for oils marketed as "classic car" or "high-zinc" if you're running something like a vintage Mercedes, old Toyota, or classic American V8.
Older diesel engines, like those in first-generation Patrol GUs or pre-2000s Land Cruisers, often run well on 15W-40 mineral diesel oil. These are simple, robust engines with tolerances designed around conventional oil. You don't need anything exotic.
How Often Should You Actually Change It?
Modern synthetics in modern engines can genuinely last 10,000-15,000 km or 12 months. Some manufacturer intervals stretch to 20,000 km.
Older engines, engines doing short trips, engines that run hot, and any engine that's been running on old or degraded oil should have it changed more frequently. 5,000-7,500 km is a safe interval for most older vehicles regardless of what the bottle says.
Oil is cheap. Engine rebuilds aren't.
Cost to buy oil: quality synthetic 5W-30 typically runs $60-90 USD ($90-135 AUD) for 5 litres. Semi-synthetic 10W-40 is usually $30-50 USD ($45-75 AUD) for 5 litres.
Cost to have it changed at a workshop: $80-150 USD ($120-220 AUD) including filter, depending on how much oil your engine takes and the oil grade required.
One Thing Worth Getting Right
The oil filter. Changing oil and leaving an old filter is a false economy. A cheap filter can collapse internally, restricting flow. Most reputable brands do perfectly fine filters. Don't buy the cheapest no-name option you can find just to save $8.
Match the filter to your engine's spec and change it with every oil change. That's all there is to it.