If we put aside all the formulas and marketing slogans, the difference between motor oils is similar to the difference between freshly brewed espresso and instant coffee. Both seem to invigorate, both perform the task, but the taste is different. It's the same here: synthetic and semi-synthetic oils work for the same purpose - engine protection, but they do it differently.
Synthetic oil is created literally "in a test tube": its molecules are structured strictly according to a given pattern, which makes the oil demonstrate stability and predictability in all conditions. Semi-synthetic, on the other hand, is a "mixed team": part of the synthetic base is mixed with mineral oil. It is cheaper, but also a compromise in terms of properties. We pay not for a beautiful canister, but for the behavior of the oil in real conditions - in the cold, in the heat, and under load.
However, it is important to understand that not every engine needs pure synthetic oil. For most mass-market cars, semi-synthetic remains a perfectly rational choice, especially if the car drives calmly and does not suffer from constant overheating or frequent starts in the cold.
Cold Start and Heat Under the Hood
Imagine a frosty morning, −25 °C, the starter is spinning briskly, and the oil, like thick honey, is trying to make its way through the channels. Synthetic oil is more fluid in such conditions - it reaches the friction pairs faster and reduces the dry start period.
At the other extreme is a summer highway, a turbine, traffic jams, and heat under the hood. Here, synthetic oil wins again: it "deflates" less from temperature, does not lose viscosity, and oxidizes more slowly. Semi-synthetic, of course, will cope, but its safety margin is lower - it ages faster, thickens, and loses its protective properties.
What's Inside: Molecules, Additives, and Compromises
Synthetic oil consists of carefully calibrated components: molecules of almost the same size form a protective film that is resistant to temperature changes. Semi-synthetic, on the other hand, is a mixture of approximately 40-50% synthetic oil with a mineral base and additives. This makes it a compromise: reliable, but not ideal in terms of stability.
Viscosity is another parameter where synthetic oil confidently leads. It holds the SAE class better and withstands a wide temperature range, remaining fluid in the cold and not thinning in the heat. This directly affects the ease of cold starting and protection in summer traffic jams. Semi-synthetic, as a rule, meets the standards, but closer to the boundaries - especially on older engines and with aggressive driving.
Engine Cleanliness and Resource
The even chemical structure of synthetic oil and high-quality detergent additives make it more resistant to oxidation. This means less carbon deposits, cleaner piston rings and channels, stable pressure and compression even by the end of the service interval.
Semi-synthetic is inferior: its additives are simpler, so the oil loses its detergent properties faster. With city driving in short bursts, degradation is especially noticeable - the oil darkens, the engine makes more noise, and consumption increases.
Oil Consumption and Level Control
Synthetic oil evaporates less at high temperatures, so you have to add it less often. This is especially noticeable on the highway and with active driving: the level remains stable, even when the turbine is working non-stop. Semi-synthetic also copes, but requires more frequent monitoring and topping up - especially on old or worn engines.
Economics of the Issue
A can of good semi-synthetic is usually two to two and a half times cheaper than synthetic with the same tolerances. Therefore, for "workhorses" like mass-market sedans or mid-class crossovers, it remains a practical solution. But for powerful turbocharged engines or premium cars, it is more reasonable to choose synthetic oil - it will ensure stable operation and protection under extreme loads.
But the main secret to engine longevity is not only what is poured in, but when. Regular oil and filter changes extend the life of the engine much more effectively than overpaying for the most fashionable brand.
Synthetic oil is a bet on maximum stability and confidence at any temperature. Semi-synthetic is a compromise between price and protection, which is quite justified if the operating mode is moderate.
Monitor the level, don't delay the replacement, choose oil according to tolerances - and then your engine will say "thank you" not with words, but with a long, trouble-free run.