Can Full Synthetic Oil Be Mixed with Semi-Synthetic Oil?
3 Answers
Whether full synthetic oil can be mixed with semi-synthetic oil depends on the composition and properties of the two oils. If they are of the same brand and model, occasional mixing is possible; mixing different brands is not recommended. The differences between full synthetic oil and semi-synthetic oil are as follows: full synthetic oil has a wider temperature range and a longer service life; for the same oil film requirements, full synthetic oil can achieve this with lower viscosity, while semi-synthetic oil requires a relatively higher viscosity to meet the same requirements. Compared to semi-synthetic oil, full synthetic oil has the following characteristics: 1. Better high and low-temperature performance; 2. Longer oil change intervals; 3. Suitable for more severe driving conditions.
I've seen many car owners mix full synthetic and semi-synthetic engine oils, but it's really not recommended as a regular practice. Last week, my relative's car had an emergency mix of 200ml semi-synthetic oil, and the engine noise became noticeably louder. The principle is simple: the two types of oil have different base oil components and additive formulations, and mixing them can easily produce sediment. In an emergency, a small amount of mixing (not exceeding 10% of the total volume) is acceptable to drive to the repair shop. However, remember to shorten the oil change interval by about 500 kilometers. If the engine gets damaged, the repair cost could buy dozens of oil barrels. It's best to do a flush during the oil change, as residual mixed oil can harm the engine.
I've conducted my own experiments, and while mixing engine oils can be used short-term, it comes with performance compromises. Full synthetic oils excel in high-temperature protection and cleanliness, while semi-synthetic oils focus on cost-effectiveness. Mixing them can lead to unstable viscosity. Especially for turbocharged German cars, using mixed oils under high-temperature conditions may cause oil film rupture. If you must mix, ensure they are the same brand and specification (e.g., both 5W-30), as mixing different brands carries greater risks. Monitor the dipstick—if the oil appears cloudy after mixing, change it immediately. For regular maintenance, sticking with full synthetic is the way to go, as it offers far superior engine protection.
