Motor oil basics
Start here if oil grades and specifications feel confusing. Plain-English answers to the concepts behind every recommendation — no jargon, no brand bias, always manual-first.
New to the terms? See the glossary or the oil viscosity chart →
Best use of this section
Use these 17 explainers to understand the words on a manual or bottle before you choose a product. For a decision, move from the concept page to the spec checker, substitution checker, or owner manual.
Current topic groups: Specifications, Basics, Myths.
- ACEA A5/B5 vs C3: differences, compatibility and risks ACEA A5/B5 compared with ACEA C3: HTHS, emissions-system compatibility, fuel economy, substitution risks and why the owner manual decides.
- API SP vs API SN: compatibility, differences and risks API SP compared with API SN: what changed, whether SP can replace SN, what backward compatibility covers, and what it does not.
- API SP vs ILSAC GF-6 API SP vs ILSAC GF-6 explained: how these recent gasoline oil standards relate, what each adds, why API SQ/GF-7 is the newer family, and why a specification is not a viscosity.
- API vs ACEA engine oil standards: what is the difference? API and ACEA oil standards compared: regions, category structure, compatibility, label reading and why one system does not automatically replace the other.
- Base oil groups explained (Groups I–V) What the API base oil groups mean, how mineral and synthetic base oils differ, and why the word 'synthetic' on a label can vary.
- Can you mix engine oils? Is it safe to mix oil brands, grades, or synthetic and conventional? What actually happens, and the manual-first rule.
- dexos1 Gen 3 vs API SP: differences and compatibility GM dexos1 Gen 3 compared with API SP: licensing, test scope, compatibility, bottle labels and why API SP alone may not satisfy a dexos requirement.
- Full synthetic vs synthetic blend The difference between full synthetic, synthetic blend, and conventional motor oil — what "blend" means, the trade-offs, and why the manual's spec and grade come first.
- How oil viscosity changes with temperature Why oil thins when hot and thickens when cold, how multigrade oils behave, what viscosity index means, and why the W and hot numbers both matter.
- Is thicker oil better? Is thicker engine oil better for your car? The short answer is no — the right oil is the grade your manual specifies, not the thickest one.
- Oil additives explained What the additive package in motor oil does — detergents, dispersants, anti-wear additives like ZDDP, viscosity modifiers, and antioxidants — and why it makes the specification matter.
- SAE oil grades explained What SAE oil grades mean — how the J300 standard defines numbers like 5W-30, and why the "W" stands for winter.
- Synthetic vs conventional oil Synthetic vs conventional motor oil explained: what each is, where synthetic helps, when conventional is fine, and why the manual still decides.
- What happens if you use the wrong oil? What happens if you use the wrong motor oil? It depends — the wrong viscosity and the wrong specification carry different, often manageable, risks.
- What is HTHS viscosity? What high-temperature high-shear (HTHS) viscosity measures, why it matters for protection under heat and load, and how it ties to specifications.
- What is LSPI (low-speed pre-ignition)? What low-speed pre-ignition is, why it mainly affects turbocharged direct-injection engines, and how modern oil specifications test against it.
- What is oil viscosity? What motor-oil viscosity means, how kinematic and dynamic viscosity differ, why temperature matters, and how an SAE grade relates to measurements.