Selecting the correct engine oil is one of the most critical maintenance decisions for any vehicle owner. Engine oil affects performance, fuel efficiency, internal component longevity, and overall engine reliability. With modern engines becoming more complex and tightly engineered, using the wrong oil can lead to premature wear, overheating, loss of power, and voided warranties.
This guide explains, in professional detail, how to choose the correct engine oil for any car.
1. Understand the Role of Engine Oil
Engine oil performs several essential functions:
• Lubrication
Reducing friction between moving parts to minimize wear.
• Cooling
Assisting the cooling system by carrying heat away from components.
• Cleaning
Removing contaminants, soot, and deposits from internal surfaces.
• Sealing
Helping piston rings seal correctly for optimal compression.
• Protection
Preventing corrosion and reducing oxidation inside the engine.
Because modern engines operate with tighter tolerances, turbochargers, high-pressure fuel systems, and advanced emissions equipment, oil formulation has become highly specialized.
2. Always Check the Manufacturer’s Specification
The most important factor is following the manufacturer’s recommended oil.
You must check:
- The vehicle owner’s manual
- Under-hood labels
- OEM certification requirements (API, ACEA, ILSAC, or manufacturer-specific standards)
Common examples of OEM standards:
- Toyota / Lexus: ILSAC GF-6, SAE 0W-20
- Honda: API SP, 0W-20
- BMW: LL-01 or LL-04
- Mercedes-Benz: MB 229.5 / 229.52
- Volkswagen / Audi: VW 502.00, 505.01
- Ford: WSS-M2C946-B1
Never select oil purely by brand name; compliance with OEM specifications is far more important.
3. Understand Oil Viscosity (SAE Ratings)
Viscosity represents the oil’s thickness and its ability to flow at different temperatures. It is indicated as:
Example: 5W-30
- 5W = winter viscosity (cold start flow)
- 30 = operating temperature viscosity
A lower first number improves cold starts and suits colder climates.
A higher second number improves high-temperature protection (useful in hot climates or older engines).
Common viscosity choices:
- 0W-20: Newer Japanese cars, high efficiency engines
- 5W-30: Universal for many cars and light trucks
- 5W-40: European cars, turbo engines
- 10W-40: Older engines, high mileage, hotter climate
4. Choose the Correct Oil Type
There are three main categories:
A. Full Synthetic Oil
- Best performance and protection
- Superior thermal stability
- Longer change intervals
- Ideal for modern engines and turbocharged units
B. Synthetic Blend
- Mix of synthetic and mineral
- Affordable with moderate performance
- Suitable for average driving conditions
C. Mineral / Conventional Oil
- Cheapest
- Suitable for older engines only
- Requires more frequent changes
Most modern vehicles must use full synthetic oil.
5. Consider Driving Conditions
Your driving style influences oil selection.
Frequent short trips
Engine does not fully warm up → needs synthetic oil with strong detergents.
Hot climate (like Cambodia)
Engines run hotter → higher high-temperature viscosity (e.g., 5W-30 or 5W-40) may be better.
Heavy loads / towing
Requires oil with stronger high-temperature resistance.
High-performance turbo engines
Require full synthetic oil meeting OEM turbo-specific standards.
6. High-Mileage Vehicles
Cars with over 120,000 km may benefit from:
- Slightly thicker oil (e.g., 5W-30 → 5W-40)
- High-mileage oil formulas (contain seal conditioners)
This helps reduce oil consumption and maintain compression.
7. Don’t Mix Oils Unless Necessary
Mixing different oil grades or brands is not recommended.
If you must top up in an emergency, use the nearest viscosity available — but perform a full change soon after.
8. How to Know If You Are Using the Wrong Oil
Symptoms include:
- Increased engine noise
- Poor acceleration
- Hard cold starts
- Oil burning or smoke
- Reduced fuel efficiency
- High engine temperature
If you experience these, change the oil immediately.
9. How Often to Change Engine Oil
Typical intervals:
- Full synthetic: 7,000–12,000 km
- Synthetic blend: 5,000–7,000 km
- Mineral oil: 3,000–5,000 km
Turbo engines or harsh conditions require more frequent changes.
10. Final Recommendation
The safest and most professional approach is:
- Follow manufacturer specifications exactly.
- Use full synthetic oil unless otherwise stated.
- Choose the viscosity based on climate and driving conditions.
- Select brands with OEM approvals (Mobil 1, Castrol, Shell, Valvoline, Liqui Moly, Idemitsu).
Doing this ensures maximum engine life, efficiency, and performance.