Synthetic vs Semi-Synthetic vs Mineral Oil: A Procurement Comparison Beyond the Marketing Label
Procurement teams often treat "synthetic," "semi-synthetic," and "mineral" as fixed technical categories. They are not. These are marketing terms whose scope varies by jurisdiction and does not, by itself, disclose the API base-oil group, viscosity index, oxidation behavior, or OEM approval of the product. The only reliable way to know what is actually in the drum is to read the technical data sheet (TDS), the certificate of analysis (COA), and the approval list — not the label.
This article breaks down what the base-oil classification system actually measures, why the word "synthetic" does not map cleanly onto that system, and what a buyer should verify before issuing a purchase order.
What Do "Mineral," "Semi-Synthetic," and "Synthetic" Actually Mean?
These three words describe a spectrum of formulation practice, not a single regulated technical standard. In practice:
- Mineral oil is typically formulated from Group I or Group II base stocks — both are refined directly from crude oil.
- Semi-synthetic (also marketed as "synthetic blend") is a blend of mineral base oil (Group I/II) with a synthetic component, in a proportion that is not universally regulated or disclosed by a fixed minimum. No industry-wide rule sets a required synthetic percentage for this label.
- Full synthetic is used commercially for products built on Group III (severely hydrocracked/wax-isomerized base oil), Group IV (polyalphaolefin, PAO), or Group V (esters, polyalkylene glycols, and other non-conventional base stocks). Critically, the chemistry underneath this single marketing word is not uniform: Group III is still crude-derived, while Group IV and Group V are synthesized or chemically re-engineered base stocks.
This is the core of the confusion: two products can both carry a "full synthetic" label while sitting in different API base-oil groups with different production routes.
How Does the Industry Actually Classify Base Oils?
The API 1509 standard (Annex E) defines five base-oil groups according to three measurable properties: percentage of saturates, sulfur content, and viscosity index (VI).
- Group I — less than 90% saturates, more than 0.03% sulfur, VI 80–119. Solvent-refined.
- Group II — at least 90% saturates, no more than 0.03% sulfur, VI 80–119. Produced by moderate hydrocracking.
- Group III — at least 90% saturates, no more than 0.03% sulfur, VI 120 or above. Produced by severe hydrocracking and wax isomerization. It is still crude-derived despite the high VI.
- Group IV — polyalphaolefins (PAO), synthesized from alpha-olefins. This group is defined by its production route rather than by the saturates/sulfur/VI test battery used for Groups I–III.
- Group V — everything else: esters, polyalkylene glycols (PAG), naphthenics, and gas-to-liquid (GTL) base stocks that fall outside the Group III specification.
A notable edge case: GTL base stocks, produced via Fischer-Tropsch synthesis followed by isomerization, are classified as Group III — not Group IV — when a given batch meets the ≥90% saturates, ≤0.03% sulfur, and VI ≥120 thresholds. Batch performance varies, so this classification should always be confirmed against the batch COA rather than assumed from the production method alone.
Because API 1509 itself is a paywalled standard, the group bands above are confirmed here through two independent trade-technical sources (Machinery Lubrication and Eurofins TestOil), both citing API 1509 and agreeing on the same boundary values. Buyers should not treat any specific VI figure (for example, a number like "123") as a spec commitment unless it appears on a batch-specific COA.
Why Doesn't "Synthetic" on the Label Guarantee a Specific Base-Oil Group?
This is a jurisdiction-specific matter of advertising law and industry precedent, not a single global technical rule.
The defining case dates to 1999 in the United States: Mobil Oil filed a complaint with the National Advertising Division (NAD) of the Council of Better Business Bureaus against Castrol, which had reformulated its Syntec product using Group III base stock instead of PAO while continuing to market it as "full synthetic." The NAD determined that Castrol had a reasonable basis for continuing to describe the Group III–based product as "synthetic."
Two things are important to understand about this finding:
- It is a determination from an advertising self-regulatory body, not a court ruling and not a government regulation. No court ever adjudicated the underlying question.
- Its practical effect has been durable: in the US and in markets that follow this precedent, a "synthetic" label does not guarantee Group IV or Group V base stock — it can legitimately describe a Group III (hydrocracked, crude-derived) product.
Other markets have not necessarily followed the same interpretation. Some markets — Germany and German-influenced markets among them — have historically taken a stricter position, reserving terms equivalent to "fully synthetic" for Group IV/V products and requiring Group III–based products to carry a "semi-synthetic" or "HC-synthesis" style description instead. This is not merely a historical tendency: German unfair-competition case law — including a 2018 ruling by the Federal Court of Justice (Bundesgerichtshof) — has treated an unqualified "vollsynthetisch" ("fully synthetic") claim on an oil containing a substantial share of Group III hydrocracked base stock as misleading, because German consumer understanding treats "fully synthetic" as a category distinct from Group III. Buyers operating in a specific jurisdiction should confirm the applicable local labeling convention — and, where a precise legal citation matters, verify the current controlling case or regulation directly — rather than assume it. For other regions, including Japan and the GCC/MENA markets, labeling conventions are not addressed here — treat labeling rules as jurisdiction-dependent and verify locally where the label matters contractually.
The procurement consequence: the word "synthetic" is a marketing and advertising-law term whose scope varies by jurisdiction. It does not, by itself, disclose the API base-oil group, the viscosity index, oxidation behavior, or OEM approval status of the product. The only reliable way to know these things is to read the TDS, the COA, and the applicable OEM or industry approval list.
What Should a Buyer Compare on Objective, Neutral Grounds?
Comparing products across mineral, semi-synthetic, and full-synthetic labels only makes sense on dimensions that can be independently checked — never on the label word itself.
- Base-Oil Group. Ask the supplier to state the API Group (I–V) directly, and confirm it against the COA rather than inferring it from the "synthetic" marketing claim.
- Viscosity Index. Group I/II base oils fall in the VI 80–119 band and Group III sits at VI ≥120 by definition. Group IV/V VI depends on the specific chemistry and is not bounded by a group-wide number. Read the actual VI from ASTM D2270, calculated from kinematic viscosity measured per ASTM D445 at 40°C and 100°C — never assume a VI figure from the label.
- Volatility (Noack). Volatility is formulation-dependent; there is no category-wide rule that "synthetic" oils are automatically lower-volatility than "mineral" oils. Measured per ASTM D5800. Where a product is licensed under the current API SQ / ILSAC GF-7 gasoline engine oil category (or the legacy API SP / ILSAC GF-6 category, still valid during the industry transition period), the specification sets a defined Noack ceiling — currently ≤15% evaporative loss under both categories. Always confirm the applicable ceiling and the actual measured result against the current spec document and the product's own TDS, since requirements can differ by viscosity grade and by which category a given product is licensed under.
- Oxidation Stability. This is additive-package-dependent, not a property that "synthetic" guarantees over "mineral" or vice versa. There is no reliable category-level claim to make here — check the TDS for the supplier's own oxidation-stability data and test method.
- Low-Temperature Performance / Pour Point. Mineral (Group I/II) oils typically need pour-point depressant (PPD) additives to reach a usable low-temperature pour point; PAO (Group IV) tends to have naturally low pour points before additisation. Measured per ASTM D97. Confirm the actual pour point on the TDS rather than assuming it from the base-oil group.
- Flash Point. Read the TDS. Measured per ASTM D92 (Cleveland open cup) or ASTM D93 (Pensky-Martens closed cup) depending on the supplier's test method — confirm which method was used, since the two are not always numerically interchangeable.
None of the above supports a blanket statement that one labeling category is "better" than another. Every dimension above must be read from supplier-provided documentation for the specific product under consideration.
What Should Procurement Verify Before Issuing a Purchase Order?
- OEM approval or licensed specification. Confirm the product carries the OEM approval or industry licence relevant to the application — for example API SN/SP/SQ, ACEA A/B and C1–C5, the relevant ACEA E-series for heavy-duty, or ILSAC GF-series licensing. Verify the licence number against the issuing body's own published list, not the supplier's claim alone.
- Read TDS values — never assume them. Confirm viscosity index (ASTM D2270), Noack volatility against the applicable spec ceiling (ASTM D5800), pour point (ASTM D97), flash point (ASTM D92 or D93), and kinematic viscosity at 40°C/100°C (ASTM D445) directly from the current technical data sheet for the specific product and batch.
- Request the batch-specific certificate of analysis (COA). A TDS describes the typical/target product; the COA documents what a specific batch actually measured. Both should be requested and cross-checked before order confirmation.
- Ask for direct base-oil group disclosure. Do not rely on "synthetic," "semi-synthetic," or "mineral" as a proxy for API Group — request the Group (I–V) explicitly and in writing.
Altonex Global operates as a discovery and RFQ venue connecting buyers with registered suppliers — it is not the seller, manufacturer, or certifier of any listed product, and cannot itself confirm a supplier's technical claims. Buyers can review lubricants listed by registered suppliers and use the RFQ process to request the TDS, COA, and OEM approval documentation directly from the supplier before committing to a purchase order.