It starts as crude petroleum and goes through refining processes like distillation and solvent refining before getting an additive package.
How Conventional Oil Is Made
Refiners turn crude petroleum into usable base oil through several steps. Distillation separates the crude into fractions, then solvent refining removes unwanted compounds. Some sources also mention hydrocracking as part of the process. Unlike synthetic oils, which are chemically synthesized from PAO or ester molecules, conventional oil keeps the natural molecular structure found in crude.
That molecular structure matters. The result is a lubricant with lower thermal stability, weaker oxidation resistance, and poorer cold-flow performance compared to synthetics.
What’s Actually Inside Conventional Motor Oil?
The base oil is mineral oil; the additive package handles the rest. Typical additives include detergents to keep engines clean, dispersants to suspend particles, anti-wear agents, antioxidants, friction modifiers, viscosity index improvers, and pour-point depressants.
These groups are less refined than the Group IV (PAO) or Group V (ester) bases used in full synthetics. That’s not a mark against them—it just explains the performance gap in extreme conditions.
When Should You Use Conventional Oil?
Conventional oil fits specific situations well. Castrol describes it as suitable for older vehicles with simple engine designs and regular driving styles. If your owner’s manual allows it, conventional oil is a solid, budget-friendly choice that protects adequately under normal duty.
For a rundown of tested options that meet common specs, see our roundup of the best conventional engine oils for your next change.
| Oil Type | Base Oil Source | Best For |
|---|---|---|
| Conventional | Refined crude petroleum (API Groups I–II) | Older engines, simple designs, normal driving |
| Semi-Synthetic | Conventional base blended with some synthetic | Budget middle ground with better protection |
| Full Synthetic | Chemically synthesized PAO or ester molecules | Severe heat, extreme cold, high-performance engines |
Common Mistakes About Conventional Oil
AAA’s synthetic vs. conventional oil research report details the performance differences under heat and cold.
References & Sources
- Castrol. “Conventional vs. Semi vs. Full Synthetic Oil.” Explains composition, use cases, and performance differences between oil types.
- AAA. “Synthetic vs. Conventional Oil Research Report.” Documents performance differences under extreme heat and cold conditions.
- Wikipedia. “Motor Oil.” Details base oil groups, hydrocarbon ranges, and additive chemistry.