Combining different types of engine lubrication fluids results in a blend that possesses characteristics intermediate to the individual components. The performance of this blend will generally fall between the performance levels of the pure synthetic and the pure conventional products. The precise outcome depends on the proportion of each type of oil and the specific additives present. For example, a 50/50 mixture will likely offer better protection than solely conventional oil, but less than a fully synthetic product.
The compatibility of these fluids allows for emergency situations where topping off the engine is necessary and the preferred lubricant is unavailable. While not ideal for long-term use, the immediate benefits of maintaining proper oil levels outweigh the marginal reduction in performance. Historically, this practice has been common due to the cost difference between the different types of products.