By Sinopec Online
Introduction
Engine oil protects moving parts, controls heat, and keeps deposits in check. Choosing between conventional and synthetic oil affects wear, fuel use, and service intervals. This guide explains the technical differences and how to choose the right oil for your engine and duty cycle.
What is conventional engine oil
Conventional oil is refined from crude and blended with detergents, dispersants, anti-wear agents, and antioxidants. It is suitable for standard driving and routine service intervals.
- Base composition: Mineral base oils from crude refining.
- Strengths: Reliable day to day performance at modest cost.
- Limits: Faster oxidation and viscosity change at high temperature. Shorter drain intervals.
What is synthetic engine oil
Synthetic oil is engineered from highly processed or chemically synthesised base stocks with very consistent molecules. It holds viscosity better, resists oxidation, and works across wide temperature ranges.
- Base composition: Group III, IV or blends with high purity.
- Strengths: Better protection under heat and load. Cleaner operation. Longer life.
- Service: Supports extended intervals when approved by the vehicle maker.
Key differences at a glance
| Feature | Conventional oil | Synthetic oil |
|---|---|---|
| Base oil type | Mineral | Engineered synthetic |
| Viscosity control over temperature | Moderate | High |
| Oxidation resistance | Lower | Higher |
| Volatility and oil consumption | Higher | Lower |
| Cold start behaviour | Acceptable | Very good |
| Typical drain interval | Shorter | Longer when approved |
| Cost per litre | Lower | Higher |
When to choose synthetic oil
- Stop start and short trips: Better protection during frequent cold starts.
- High load or towing: Resists oxidation and viscosity rise at temperature.
- Modern engines: Many designs expect low volatility and stable viscosity to meet emissions and fuel economy targets.
- Extended service: When the vehicle maker allows longer intervals, synthetic oil helps hold condition between services.
When conventional oil remains suitable
- Older engines with simple duty and short service intervals.
- Vehicles driven in mild climates at steady speeds.
- Situations where cost per litre is the main constraint and the service schedule is frequent.
Viscosity grade still matters
Always use a viscosity grade that meets the handbook specification. A premium synthetic in the wrong grade can still underperform. Check ambient temperature range, load, and any maker approvals that apply to your engine.
Switching from conventional to synthetic
- Confirm the correct viscosity and performance level in the handbook.
- Change the oil and filter. Inspect for leaks and monitor consumption during the first interval.
- For high mileage engines with deposit history, shorten the first synthetic interval and check the filter for loading.
Common myths
- Myth: Synthetic oils are always thin. Fact: They are available in the same viscosity grades but hold grade more reliably with temperature.
- Myth: You cannot mix conventional and synthetic. Fact: Emergency top up is acceptable, but best results come from a full change to one type.
Cross references
- Group I vs Group II base oils
- Group II vs Group III base oils
- Why OEM approved engine oils matter for modern fleets
Published by Sinopec Online Technical Team
Frequently asked questions
How often should I change synthetic oil
Follow the vehicle maker schedule. Extend only when the handbook allows and the duty cycle supports it.
Can I switch back to conventional after using synthetic
Yes, if the viscosity and performance level meet the handbook. Staying with synthetic preserves the benefits of stability and low volatility.
