Can Fully Synthetic Oil and Semi-Synthetic Oil Be Mixed?

[McBarrett]

Fully synthetic oil and semi-synthetic oil cannot be mixed. The methods for changing engine oil are as follows: 1. The functions of engine oil include: lubrication, auxiliary cooling, cleaning, sealing and leak prevention, rust and corrosion prevention, shock absorption, and anti-wear. Engine oils are categorized into mineral oil, semi-synthetic oil, and fully synthetic oil. 2. Fully synthetic oil: Fully synthetic oil is entirely produced through chemical processes that decompose various natural substances and then synthesize them with other materials. The advantages of fully synthetic oil include high cleanliness, strong fluidity, effective cooling, excellent oxidation resistance, and a longer replacement interval, typically around 1 year or 10,000 kilometers. Vehicles with turbocharged engines are recommended to use fully synthetic oil. 3. Semi-synthetic oil: Semi-synthetic oil is a blend of synthetic oil and mineral oil, with a ratio of 4:6 between mineral oil and synthetic oil. The characteristics of semi-synthetic oil include possessing the advantages of synthetic oil while being cheaper than fully synthetic oil. However, its performance in all aspects is weaker than fully synthetic oil, with the only advantage being its lower price. The replacement interval is also slightly shorter than that of fully synthetic oil, around 7,000-8,000 kilometers or 8 months. Semi-synthetic oil is more suitable for naturally aspirated engines.

[LaBriella]

As a veteran technician at a repair shop, I've seen many car owners mix full synthetic and semi-synthetic oils. These two types of oil have different base oils and their additive formulations aren't fully compatible. Mixing them may cause chemical reactions leading to precipitation, which affects lubrication performance.

For short-distance emergency use, mixing once isn't a big problem - like adding half a liter of full synthetic to semi-synthetic oil won't immediately damage the engine. However, long-term mixing is definitely not advisable as it alters oil viscosity, reduces protective capabilities, and accelerates engine wear.

I've handled cases where long-term mixing caused sludge to clog oil passages, making cleanup particularly troublesome. I recommend everyone choose oil according to their vehicle's manual requirements, and when changing oil, always completely drain the old oil before adding new oil.

[BrynleeRose]

As a car enthusiast with over a decade of modification experience, my fellow car lovers and I always try to avoid mixing different engine oils. Fully synthetic and semi-synthetic oils are like different brands of engine oil, with significant differences in their chemical formulations. Mixing them can easily lead to impurities. It's understandable to add some in an emergency during long trips when oil is low, but it's best to change it within 500 kilometers. After mixing, I noticed the engine noise increases, and fuel consumption goes up by a few tenths, indicating reduced lubrication effectiveness. If you must mix, choosing the same brand and viscosity grade is slightly better, such as mixing 5W-30 semi-synthetic with the same fully synthetic oil. However, now that fully synthetic oil prices have dropped, there's really no need to save money on this.

[Hayley]

From an automotive engineer's perspective, engine oil formulation is a precise system. Full synthetic base oils have higher purity, while semi-synthetics contain mineral oil components. Mixing them disrupts the additive balance. The most common issue is viscosity index improver failure, leading to insufficient high-temperature protection. In engines where mixed oils were used, I've observed significantly increased piston ring carbon deposits. That's why manufacturer maintenance manuals prohibit oil mixing, even within the same brand. If accidental mixing occurs, shortening the oil change interval is recommended. For regular maintenance, full synthetic oils are actually more cost-effective as their change intervals are twice as long as semi-synthetics.