Full synthetic vs synthetic blend: what the labels actually mean

Three oils, one shelf

Every motor oil is two things: a base oil, which is most of the jug, and an additive package, which is the detergents, anti-wear agents, and viscosity modifiers that do the specialized work. The words on the front of the jug describe the base oil.

• Conventional is refined directly from crude. The molecules come in whatever sizes and shapes the crude had, which means some of them break down or evaporate sooner than others.
• Full synthetic is a base oil that has been chemically rebuilt into uniform molecules, either by severe processing of petroleum (the most common route) or by building it from smaller chemicals. The result is more consistent, flows better cold, and resists heat and oxidation longer.
• Synthetic blend is conventional oil with some synthetic mixed in.

The part nobody puts on the label

There is no rule for how much synthetic a "synthetic blend" has to contain. A blend can be a quarter synthetic, or it can be a splash. The word tells you it is better than conventional, and it usually is, a little. It does not tell you by how much, and the price gap between conventional and blend is often larger than the performance gap.

"Full synthetic" in the United States is also broader than it sounds. Since a 1999 advertising ruling, heavily processed petroleum base oils (the industry calls them Group III) can be sold as synthetic alongside the lab-built kinds (Groups IV and V). That upset purists at the time. In practice it has not mattered much to drivers, because modern Group III synthetics perform very well, and because what actually matters is the next section.

What actually matters: the specification

Your engine does not care how the base oil was made. It cares whether the finished oil passes the tests your manufacturer requires. Those tests are the letters on the back of the jug: API SP, ILSAC GF-6A, and manufacturer approvals like GM's Dexos1 Gen 3. A blend that carries the right approval is a legitimate choice. A "full synthetic" that does not carry the approval your manual asks for is the wrong oil, however it was made.

That said, the newest specifications are hard to pass. API SP and Dexos1 Gen 3 both test for low-speed pre-ignition, timing chain wear, and high-temperature deposits, and in practice almost everything that passes them is a full synthetic. Many newer turbocharged and direct-injection engines simply require full synthetic. Check the oil cap or the manual. If it says "full synthetic" or lists a Dexos number, the choice is already made.

Where full synthetic earns its price

Cold starts in January. Flows at low temperatures where conventional oil thickens. Most engine wear happens in the first minute after a cold start.

Turbocharged engines. A turbo bearing runs far hotter than the rest of the engine. Conventional oil cooks onto it. Synthetic resists forming deposits.

Long intervals. Synthetic resists oxidation and thickening, so it stays in spec for 7,500 to 10,000 miles where conventional is done at 3,000 to 5,000.

Oil consumption. Uniform molecules evaporate less at high temperature, so the level drops more slowly between changes.

Towing, idling, stop and go. Heat and fuel dilution break conventional oil down fast. Synthetic tolerates both longer.

The cost question, done honestly

A conventional oil change is cheaper on the day. It is not usually cheaper per mile. If conventional gets you 4,000 miles and full synthetic gets you 8,000, you are buying two conventional changes for every synthetic one, and paying twice for the filter, the disposal fee, and your time. For most drivers full synthetic costs about the same per year and the engine is better protected the whole time. For a turbocharged or direct-injection engine there is no real choice to make.

The one case where a blend or conventional makes sense is an older, simple engine that consumes oil quickly anyway, where the oil is not staying in long enough for the synthetic advantage to show. Even then, the cold-start protection is a real benefit here in Iowa.

Why we only carry one

Oil To Go carries full synthetic only, licensed to API SP, ILSAC GF-6A, and Dexos1 Gen 3. Partly that is because it is the right oil for nearly everything we see. Mostly it is because carrying one oil means one tank in the van, one flat price, and no "would you like to upgrade" conversation in your driveway. The choice is made before we arrive, and it is the good one.