Quick answer
Conventional motor oil is refined directly from crude (Group I or II base stock). Full synthetic oil is either hydrocracked petroleum re-engineered into more uniform molecules (Group III) or chemically synthesized from gas (Group IV PAO) or esters (Group V). Synthetic resists shear, oxidation, and cold thickening dramatically better and is now required by most 2010+ engines.
The five API base-oil groups
- Group I - solvent-refined, less than 90% saturates. Cheap, used in industrial lubricants. Almost no passenger-car motor oil uses it anymore.
- Group II - hydrotreated, 90%+ saturates. Still legally "conventional." Most "regular" 5W-30 on the shelf today is Group II.
- Group III - severely hydrocracked, very high saturates, VI > 120. Marketed as "synthetic" in North America since a 1999 BBB ruling. Mobil 1, Castrol Edge, Pennzoil Platinum all use it.
- Group IV (PAO) - polyalphaolefins, chemically synthesized. True synthetic. Used in premium products like Mobil 1 ESP and some Amsoil grades.
- Group V - esters, alkylated naphthalenes, etc. Premium additive blend stocks; rarely the dominant base in passenger oil.
Performance differences that matter
| Property | Conventional | Full synthetic |
|---|---|---|
| NOACK volatility (lower=better) | 12-18% | 5-10% |
| Pour point | -25 to -35°C | -40 to -55°C |
| Viscosity index | 95-120 | 140-180 |
| Oxidation stability | Moderate | 2-3x longer |
| Typical change interval | 3,000-5,000 mi | 7,500-15,000 mi |
| Cost per quart (2026) | $5-7 | $8-12 |
When synthetic is required (not optional)
- Any engine with a turbocharger - synthetic resists coking on hot turbo bearings.
- Any engine with direct injection. Conventional oil's NOACK volatility contributes to intake-valve carbon buildup.
- Any engine spec'd for 0W-20 or 0W-16 - the cold-flow rating is only achievable with synthetic base stock.
- European brands with extended-drain intervals (15,000-20,000 mi). VW 504/507, MB 229.5, BMW LL-04 all require approved synthetic.
When conventional is fine
Conventional 5W-30 or 10W-30 still works perfectly in older non-turbo engines (pre-2000s small-block V8, port-injected straight-sixes, older 4-cylinders) IF you change it every 3,000-5,000 miles. The shorter interval makes up for the lower oxidation resistance.
Conventional is also reasonable for vehicles that sit a lot - the oil oxidizes more from time than from miles, and you'll change it on the calendar anyway.
Cost per mile (the only number that matters)
A 5-quart jug of conventional 5W-30 at $30 changed every 4,000 miles = $0.0075/mi. A 5-quart jug of full synthetic at $45 changed every 8,000 miles = $0.0056/mi. Synthetic is roughly 25% cheaper per mile, ignoring filter cost.
Add fuel-economy gains (1-2% for synthetic in some applications) and synthetic is the better deal in almost every modern car.
Common mistakes
- Believing the "you can never go back" myth. Switching from conventional to synthetic (or vice versa) is harmless. The base-oil chemistry is fully compatible.
- Paying for synthetic and then changing it at 3,000 miles. You wasted 50% of its life. Synthetic earns its premium at 7,500-10,000 miles.
- Using synthetic blend (semi-synthetic) and assuming it has full-synthetic performance. Most blends are 70-80% Group II conventional with a Group III booster. Real-world performance is closer to conventional than synthetic.