"Synthetic is better" is true and useless. Better at what, and does that thing matter for your engine? Here are the six properties that actually differ, what each one is measuring, and when it changes your decision.

  • The differences are measurable properties, not marketing categories.
  • Pour point and volatility matter most in cold climates and hot, hard-working engines.
  • Oxidation stability is what determines whether an interval is safe to extend.
  • If none of the six apply to how you drive, mineral oil in the correct grade is not a mistake.

1. Base stock uniformity

Mineral oil is a mixture of molecule shapes left over from refining crude. Synthetic is built to a single molecular structure. Every difference below is a downstream consequence of this one.

2. Pour point

The lowest temperature at which the oil still flows. Synthetics pour far colder. If you start a cold engine in winter, this is the property you are buying - oil that reaches the valvetrain in seconds instead of tens of seconds.

3. Volatility

How much oil boils off as vapor when hot, measured as NOACK volatility. Mineral oil contains light molecules that evaporate, which is why conventional-oil engines tend to consume more between changes and leave more behind on hot surfaces.

4. Oxidation stability

Oxidation is oil reacting with oxygen under heat, and its end products are sludge and varnish. Synthetic resists it substantially longer. This is the single property behind extended drain intervals - and behind why turbo engines are so specific about oil.

5. Viscosity index

How much the oil thins as it heats. A higher viscosity index means more stable behavior between a cold start and a hot highway run. Synthetics run higher, so the protective film thickness varies less across the range the engine actually operates in.

6. Cost per mile

Synthetic costs roughly two to three times more per quart, but is often run at longer intervals. Per mile the gap narrows considerably - and for many drivers the real cost difference over a year is smaller than one tank of fuel.

When the six do not matter

An older, naturally aspirated engine, in a mild climate, driven regularly, changed on a short interval, with a manual that calls for conventional oil: the properties above are all real and none of them are binding. Buy the correct grade and change it on time. That matters more than the base stock.

Turbocharged engine, cold winters, or an interval past 7,500 miles - if any one of those describes you, the six differences above stop being academic and synthetic is doing real work.

Keep reading

Start with the plain-language synthetic vs mineral comparison, decode grades in viscosity grades explained, or see how 5W-30 synthetic protects an engine.

Keep learning: Motor oil, explained · Viscosity grades · Oil change intervals