The three base oil types
The terms full synthetic, synthetic blend, and conventional describe the base oil an engine oil is built on, not its viscosity or its specification.
- Conventional oil uses base stocks refined from crude oil.
- Full synthetic oil uses base stocks that are chemically engineered for more uniform and predictable molecular structure.
- Synthetic blend, sometimes called semi-synthetic, mixes synthetic and conventional base oils together.
All three are then combined with an additive package to make a finished product. So “synthetic” describes the foundation of the oil, while the grade and specification describe how it is rated and what it has been tested to do.
What “blend” means and the trade-offs
A synthetic blend is exactly what the name suggests — a mixture of synthetic and conventional base oils. It is not a full synthetic, and the proportion of synthetic is not standardised across brands. It generally sits between conventional and full synthetic in both performance characteristics and cost.
Broadly, full synthetics tend to offer more consistent performance and are often used where engines or service intervals demand it, but they usually cost more. Conventional oils cost less and remain suitable for many engines. A blend aims to balance the two. These are general tendencies, not guarantees, because the additive package and the standards an oil meets also shape real-world performance.
Let the manual decide
The base oil type is not the first thing to choose. The first thing is the specification and grade your owner’s manual lists for your engine — for example a particular SAE grade together with an API, ILSAC, ACEA, or OEM standard.
Once you know the required grade and specification, you can pick a conventional, blend, or full synthetic product that meets them. Some engines specifically require a full synthetic; others do not. Following the manual ensures the oil is appropriate, rather than assuming that a more expensive base oil type is automatically the better answer for your engine.