What Each Oil Actually Is
Conventional motor oil is refined directly from crude petroleum. The molecules are uneven in size and shape, which means the oil breaks down faster under heat and loses viscosity more quickly under shear stress. Full synthetic oil starts with a similar base but goes through additional chemical processing that produces uniform molecules engineered for specific performance properties. The result is an oil that stays in grade longer, flows better in cold weather, and resists thermal breakdown.
There's also a middle option, synthetic blend, which mixes the two. It costs less than full synthetic but doesn't carry the full performance benefit. For most drivers, it makes more sense to pick one side or the other.
Cost and Change Intervals
On a single visit, conventional is cheaper: roughly $35–$50 versus $50–$75 for full synthetic at a typical shop. But the real comparison is annual cost. Synthetic oil is engineered for a 7,500–10,000-mile change interval, while conventional typically maxes out at 3,000–5,000 miles.
A driver putting 15,000 miles per year on conventional oil will need three to five changes annually, totaling $105–$250. On synthetic, that same driver needs only one or two changes per year, totaling $100–$150. Over the life of a vehicle, synthetic often ends up cheaper, with less time spent in the waiting room.
Which Engines Require Synthetic
Many modern engines are not approved for conventional oil at all. The requirement usually applies to:
- Turbocharged engines — the turbo bearing runs at temperatures that shear conventional oil rapidly. - Direct-injection engines — fuel dilution and carbon deposits require synthetic's detergent package. - Most vehicles built 2010 or later — tighter emissions and efficiency targets drove manufacturers toward synthetic-only specs. - European luxury brands — BMW, Mercedes, Audi, and Volvo have specified full synthetic for two decades.
Check your owner's manual. If it calls for an oil meeting API SP, dexos1 Gen 3, MB 229.5, BMW LL-01, or similar modern specifications, conventional oil won't meet the spec and could trigger engine damage over time.
Cold-Weather Behavior
If you live anywhere with winter temperatures dipping below freezing, synthetic's advantage grows dramatically. Conventional oil thickens as temperatures drop, which means the oil pump has to work harder to circulate it at startup — the moment when most engine wear actually happens. Synthetic maintains its flow characteristics down to -40°F, reaching critical bearings and valvetrain components within seconds. For drivers in northern climates, this alone justifies the switch.
Is Synthetic Worth It?
For newer vehicles, the answer is essentially forced: you need synthetic to meet the spec. For older vehicles where either is acceptable, synthetic is still usually the better value once you account for fewer changes per year, less engine wear, and cleaner internals at the 150,000-mile mark. The only scenarios where conventional still makes sense are older pre-2010 engines with short city commutes where you change oil on a time-based schedule anyway, and certain fleet or heavy-use applications where oil is changed frequently regardless.