Fully Synthetic vs Semi-Synthetic vs Mineral Engine Oil: What Is the Difference?
Every engine oil is made from base oil plus an additive package. The type of base oil is what the words fully synthetic, semi-synthetic and mineral describe. It affects how the oil copes with cold starts, high temperatures and long service intervals.
The three types at a glance
| Type | What it is | Typical grades | Best for |
|---|---|---|---|
| Mineral | Base oil refined directly from crude oil. | 15W-40, 20W-50 | Older and classic engines, and some heavy-duty diesels, where the handbook allows it. |
| Semi-synthetic | A blend of mineral and synthetic base oils. | 10W-40 | Older petrol and diesel engines with fixed service intervals. |
| Fully synthetic | Highly refined or chemically built base oils with very consistent molecules. | 0W-16 to 5W-40, plus 10W-60 | Almost all modern engines, and any engine on long-life servicing. |
Why fully synthetic oil performs better
- Better cold flow. It reaches moving parts faster on a cold start, when most engine wear happens.
- Better heat resistance. It resists thickening and breaking down at the high temperatures found in turbochargers.
- Lower volatility. Less oil evaporates, so you top up less often.
- Longer service life. Long-life approvals such as VW 504 00 / 507 00 and BMW Longlife-04 are met by synthetic oils.
Which type does my car need?
Your handbook decides it, not the label on the front of the bottle. Most modern specifications, including ACEA C2, C3 and C5 and the major long-life approvals, are met only by fully synthetic oils. If your handbook lists a grade such as 10W-40 with ACEA A3/B4, a semi-synthetic oil meeting that specification is suitable. See our guide to choosing the right engine oil.
Can I switch from mineral to synthetic oil?
Yes, as long as the synthetic oil meets the grade and specification your engine needs. Switching does not harm an engine in good condition. Going the other way, from synthetic to semi-synthetic or mineral, is only fine if that oil still meets your handbook's requirement.
Is fully synthetic oil worth the extra cost?
For modern engines it is usually required anyway. For older engines it can still help on cold starts and with oil consumption, but a semi-synthetic or mineral oil that meets the handbook specification will do the job at a lower price.
Shop by type of engine
- 5W-30 engine oil and 0W-20 engine oil for modern cars.
- 10W-40 engine oil for older petrol and diesel engines.
- 15W-40 and 20W-50 for older and classic engines.
Frequently asked questions
Does fully synthetic oil last longer?
It resists breaking down for longer, which is why long-life service schedules need it. Always change it at the interval in your service schedule, not later.
Will synthetic oil cause leaks in an older engine?
No, synthetic oil does not damage seals. An engine that already has worn or hardened seals may leak whatever oil you use.
Is semi-synthetic oil the same as part-synthetic?
Yes, the two names mean the same thing: a blend of mineral and synthetic base oils.