Choosing the right engine oil seems simple until you look at the shelf and see dozens of options with different numbers, letters, and marketing claims. Using the wrong oil can cause accelerated engine wear, increased oil consumption, or poor cold-weather performance. This guide explains what the numbers and specifications actually mean so you can make an informed choice.
What the Viscosity Numbers Mean
The numbers on an oil bottle (like 5W-30 or 0W-20) describe the oil’s viscosity – its resistance to flow. The first number followed by “W” (which stands for Winter) indicates the oil’s flow characteristics at cold temperatures. Lower numbers flow more easily in the cold, which means faster lubrication at startup. The second number indicates viscosity at operating temperature (100 degrees Celsius). Higher numbers mean thicker oil at operating temperature.
A 0W-20 oil flows very easily when cold (good for cold starts and fuel economy) and is relatively thin at operating temperature. A 10W-40 is thicker when cold and thicker when hot. Modern engines are designed around specific viscosity grades, and using a different grade than specified can affect oil pressure, fuel economy, and engine wear.
Performance Specifications
The viscosity grade alone is not enough. You also need to match the performance specification, which defines the additive package and quality level of the oil. The main specifications you will encounter are API (American Petroleum Institute) ratings like SN Plus or SP for petrol engines and CK-4 for diesels, ILSAC ratings like GF-6A and GF-6B which are common for Japanese manufacturers, ACEA ratings like A3/B4 or C3 which are common for European vehicles, and manufacturer-specific approvals like VW 504.00/507.00, BMW LL-04, or Mercedes-Benz 229.52.
Your workshop manual specifies both the viscosity grade and the performance specification required. These two pieces of information together define exactly which oils are suitable for your engine. Using an oil that meets the viscosity grade but not the performance specification (or vice versa) is not correct.
Mineral vs Synthetic vs Semi-Synthetic
Mineral oil is refined from crude petroleum and is the cheapest option. It is adequate for older engines with wider tolerances but breaks down faster at high temperatures. Full synthetic oil is chemically engineered to provide superior protection, better cold-flow properties, longer service life, and better resistance to thermal breakdown. Semi-synthetic (or synthetic blend) is a mix of both and offers a middle ground in terms of price and performance.
Most modern engines, especially those with turbochargers, direct injection, or variable valve timing, require fully synthetic oil meeting a specific performance standard. Check your workshop manual rather than assuming.
Why “Universal” Oils Are Risky
Some oil brands market their products as suitable for “all vehicles” or “all Japanese cars”. While these may technically meet a broad range of minimum specifications, they may not meet the specific manufacturer requirement for your engine. A Toyota requiring 0W-20 ILSAC GF-6B and a Subaru requiring 0W-20 API SN Plus have different additive requirements despite sharing the same viscosity grade. Always match the exact specification in your manual.
Your Manual Is the Definitive Source
The correct oil grade, performance specification, and capacity are listed in your vehicle’s workshop manual. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
