Synthetic and fully synthetic oil are often used interchangeably, but "fully synthetic" is the more precise term. Fully synthetic oil is engineered from chemical compounds built from scratch in a laboratory, creating uniform molecules designed for optimal performance. Synthetic oil, sometimes called "semi-synthetic" or "synthetic blend," mixes conventional mineral oil (refined from crude) with synthetic base stocks, typically at a ratio of 30–50% synthetic to mineral oil.
The key practical differences stem from their composition. Fully synthetic oil offers superior performance across all metrics: it flows better at extremely cold temperatures, resists breakdown at high heat longer, provides better lubrication and engine protection, and lasts significantly longer between oil changes—often 10,000 miles or more compared to 5,000–7,000 miles for mineral oil. It also maintains consistent viscosity over wider temperature ranges and produces fewer emissions.
Synthetic blends offer a middle ground—improved performance and longevity compared to mineral oil, but not as extensively as fully synthetic. They're less expensive than fully synthetic while still providing benefits like better cold-start flow and extended change intervals (typically 7,500 miles).
Which type you should use depends on your vehicle's specifications. Check your owner's manual, as some modern engines are designed for and require fully synthetic oil, while others run fine on conventional mineral oil. Using the correct grade matters more than choosing synthetic over mineral; using the wrong viscosity can harm engine performance.