What Oil Does My Car Need? How to Find the Right Engine Oil

By Salek Tyre & Mechanic · Ashford TW15

Engine oil is the lifeblood of your vehicle. It lubricates moving parts, reduces friction, cools the engine, and cleans away soot and deposits. However, using the wrong type of engine oil can lead to poor fuel economy, increased wear, and potentially catastrophic engine damage. Here is how to find the exact oil your car requires.

1. Finding your recommended engine oil type

Engine oil requirements are specific to your vehicle's engine design. To find the correct oil specifications, consult these sources:

• The Vehicle Handbook: The owner's manual contains a section on vehicle fluids, listing the required oil viscosity (e.g. 5W-30) and the specific manufacturer standard (e.g., VW 507.00, Ford WSS-M2C913-D).
• The Oil Filler Cap: Lift the bonnet and inspect the oil filler cap. Some vehicle manufacturers (especially BMW, Castrol, and Ford partners) engrave the recommended viscosity grade directly on it.
• Online Reg Lookup Tools: Many major oil manufacturers provide registration number lookup tools on their websites that recommend compatible oils.
• Ask a Professional: If you are unsure, contact us at Salek Tyre & Mechanic with your registration number, and we will look up the manufacturer specification for you.

2. Viscosity grades explained: What do the numbers mean?

Most modern engine oils are "multigrade" oils, meaning they are designed to perform well in both freezing winter temperatures and high operating temperatures. These are represented by codes like 5W-30 or 10W-40:

• The Cold Number (e.g. 5W): The first number followed by a "W" (which stands for Winter) indicates the oil's viscosity (thickness) at low temperatures. A lower number means the oil flows better in cold weather, allowing the engine to start easily and protecting internal components from wear on cold starts.
• The Hot Number (e.g. 30): The second number indicates the oil's viscosity at standard engine operating temperatures (100°C). A higher number means the oil remains thicker at high temperatures, offering stronger film protection under heavy load or hot conditions.

5W-30 vs 5W-40: What's the difference?

Both oils start with a cold rating of 5W, meaning they flow identically in cold winter conditions. However, at operating temperatures, 5W-40 remains thicker (more viscous) than 5W-30. If your engine is designed for a thinner oil like 5W-30 to improve fuel efficiency, putting in a thicker 5W-40 can make the engine work harder and decrease fuel economy. Conversely, if your car requires 5W-40 for high-temperature protection, using 5W-30 may result in insufficient lubrication film strength when the engine is hot.

3. Mineral, Semi-Synthetic, and Fully Synthetic Oil

In addition to viscosity, oils are categorized by how they are manufactured:

• Fully Synthetic Oil: Chemically engineered in a laboratory for consistent molecular structure. Fully synthetic oils offer the best protection, flow better in the cold, resist thermal breakdown at high temperatures, and last longer. Almost all modern cars built after 2010 require fully synthetic oil.
• Semi-Synthetic Oil: A blend of synthetic and mineral oils. It offers better protection than pure mineral oil at a lower price point than fully synthetic. Often used in slightly older cars or moderate-displacement engines.
• Mineral Oil: Refined crude oil. It provides basic lubrication but breaks down faster under heat and offers less cold-start protection. Mainly used in classic cars or older engines designed before synthetic oils became standard.

4. Why manufacturer specifications matter

Viscosity (like 5W-30) is only half the story. Vehicle manufacturers have unique engine designs (such as turbos, particulate filters, and direct injection systems) that require specific chemical additives in the oil. These standards are listed on the back of the bottle (e.g., ACEA C3, API SP, VW 504.00/507.00). Using an oil with the correct viscosity but the wrong manufacturer specification can cause oil sludge, block diesel particulate filters (DPFs), or accelerate wear on timing chains.

5. What happens if I use the wrong oil?

• Increased Fuel Consumption: Using an oil that is too thick increases friction within the engine, requiring more fuel to run.
• Engine Wear: Using an oil that is too thin means the protective film between moving metal parts can break down, leading to accelerated wear.
• Component Damage: Modern exhaust emissions systems, like catalytic converters and particulate filters, can be quickly ruined by oil with incompatible additive levels.
• Voided Warranty: Using oil that does not meet the manufacturer's specified standards can void your vehicle's warranty.