Can Fully Synthetic and Semi-Synthetic Engine Oils Be Mixed?

[MacRosalie]

Fully synthetic and semi-synthetic engine oils cannot be mixed. The functions of engine oil include: 1. Lubricating pistons, piston rings, and cylinders; 2. Lubricating intake valve stems; 3. Reducing engine friction and high temperatures; 4. Cleaning carbon deposits, sludge, and worn metal particles. The engine oil replacement process involves: 1. Removing the drain plug to discharge used oil; 2. Removing the oil filter; 3. Wiping the filter housing and replacing the oil filter element; 4. Adding new engine oil; 5. Checking for oil leaks. The hazards of mixing different grades of engine oil include: 1. Abnormal smoke from the exhaust pipe; 2. Accelerated sludge accumulation; 3. Reduced sealing performance of piston rods or piston rings, leading to discoloration or damage; 4. Engine malfunctions; 5. Severe wear of pistons and cylinder walls.

[LaAlessandra]

I've been repairing cars for over a decade and often see car owners mixing full synthetic and semi-synthetic oils to save money. Let me put it this way: an engine is like human blood vessels. The two oil formulations differ, and mixing them messes up viscosity while reducing cleaning effectiveness. Last week, a regular customer mixed oils and developed sludge in just 2,000 km - it cost him over 800 yuan to dismantle and clean. In emergencies, you can add some semi-synthetic as a temporary fix, but never exceed half a liter, and immediately drive to a repair shop for complete replacement. Remember: always use either full synthetic or semi-synthetic oil during maintenance - mixing saves no money and ruins your engine.

[VanElliot]

Trying to save money on DIY car maintenance, right? I've done the mix-and-match thing before—pouring leftover full synthetic oil into a semi-synthetic bottle for reuse. The result? Increased engine noise and a half-point rise in fuel consumption. A mechanic explained that mixing oils can cause additive conflicts, with some components precipitating into sludge. Even though manufacturers claim compatibility, the molecular structures of base oils differ. If you really want to save, stick to frequent changes of regular semi-synthetic oil or stock up on the same brand and series during sales—mixing isn’t worth the trade-off.

[StKingston]

From a chemical perspective, full synthetic oil consists of polyalphaolefin (PAO) base oil with precision additives, while semi-synthetic oil blends mineral oil with Group III base oils. Mixing them may lead to excessive zinc-phosphorus anti-wear additives or deactivation of acid neutralizers. Short-term effects might only accelerate oil oxidation, but prolonged use (over six months) can significantly cause varnish deposits. If a transition is unavoidable, mixing old and new formulations from the same brand is safer than cross-brand mixing. Remember to shorten the oil change interval by 25% after mixing for optimal safety.

[LeIsaac]

Our fleet of over 50 vehicles never dares to mix engine oils. Last year, a driver mixed semi-synthetic oil for convenience, resulting in turbocharger bearing damage within just three months, with repair costs equivalent to a decade's worth of oil expenses. Different oil formulations vary significantly in shear resistance, and mixing them can thin the high-temperature oil film, posing greater harm, especially to newer engines with start-stop functionality. Always use the exact oil grade specified on the engine cover, and if there's leftover oil, store it for topping up the same type rather than using it in the next maintenance.