Semi-Synthetic Vs. Synthetic Blend Oil: Are They The Same Thing?

Motor oils are broadly categorized into fully synthetic, conventional, and synthetic blends (also called semi-synthetic or part synthetic), with semi-conventional and high-mileage full synthetics being other notable variations. It's confusing, and it's no wonder drivers all over the U.S. have questions. Should you settle for semi-synthetic? Should you use full synthetic or high-mileage engine oils? Are semi-synthetic and synthetic blends the same? While there's a clear distinction between mineral and fully synthetic oils, the same can't be said about semi-synthetic and synthetic blends — they're virtually identical.

Understand that engine oils, regardless of their type, are formulated using two main components — base oils and additives. The various classifications largely boil down to the base oil composition. Put simply, the base oils in conventional motor oil are 100% crude oil-derived, hence why they're sometimes called mineral oil. Fully synthetics, on the other hand, use chemically synthesized base oils, while semi-synthetics use a mixture of both.

It's worth pointing out that full synthetics also have roots in crude oil, but they are further engineered to weed out the undesirable molecular properties otherwise seen in mineral oils. Although semi-synthetics sit in a happy middle ground, there's no hard-and-fast rule regarding the mineral-to-synthetic oil ratio. There are, however, a few indicators that might tell you how much "synthetic" is in the oil.

The amount of processing base oils go through can affect their cost and quality. So naturally, full synthetics tend to be more expensive than conventional mineral oils, while semi-synthetics sit somewhere in between. However, it's worth knowing that certain lesser-known brands sell bottles labeled "synthetic" when, in fact, there's barely any synthetic base oil in them. To avoid falling for this, check the manufacturer's website (if they have one) and look for viscosity index and pour point in the product data sheet. Oils with a higher viscosity index and lower pour point tend to have a better synthetic base oil.

Semi-synthetic blends are in a difficult spot today, as manufacturers struggle to adapt these oils to meet the ever-growing efficiency and performance demands of new-age engines. But no matter the type, or dare we say, whether you choose name brand or store brand engine oils, modern blends are incredibly sophisticated. Even if you went with mineral instead of synthetic during an emergency top-up, you'd still be fine so long as you mix the same API grades (SP, SN, SM, etc.). But make sure you double-check the owner's manual for the engine oil viscosity — you don't want to screw that up.