Are all "fully synthetic" oils the same?

Walk into any motorcycle shop in Singapore or Kuala Lumpur and you will find a dozen oils labelled "fully synthetic." The prices range from $8 to $35 per litre. The base API rating is often the same. The grade is the same. The label says "full synthetic" on all of them. They are not the same product. The difference is in the base oil group and the additive package — and understanding those two things tells you what you are actually buying.

What "fully synthetic" actually means — and why it is complicated

The term "fully synthetic" has no single technical definition. It is a marketing term. In 1999, the US National Advertising Division of the Better Business Bureau broadened the legally permissible use of "synthetic" to include Group III base stocks — highly refined mineral oils produced by hydrocracking crude petroleum. Before that ruling, only Group IV and Group V base oils (true synthetics, chemically engineered rather than refined from crude) could be called synthetic.

The result: two products can both carry "fully synthetic" on the label. One is manufactured from crude oil that has been very highly refined. The other is chemically synthesised from petrochemical feedstocks. Both are legal. They are not the same.

The five base oil groups

The American Petroleum Institute classifies base oils into five groups based on chemical composition and properties:

Group I (solvent refined): Sulphur >0.03%, saturates <90%, Viscosity Index 80–120. The most basic refining process. Still widely used in conventional oils.

Group II (hydrotreated): Sulphur ≤0.03%, saturates ≥90%, VI 80–120. Cleaner than Group I but still a mineral oil.

Group III (hydrocracked): Sulphur ≤0.03%, saturates ≥90%, VI ≥120. Very highly refined mineral oil. Legally permitted to be called "synthetic" since 1999. This is what most mid-priced "fully synthetic" motorcycle oils use.

Group IV (PAO — Polyalphaolefin): True synthetic base oil, chemically engineered by polymerising ethylene. Viscosity Index typically 140–175 depending on grade. Molecules are uniform, pure, and purpose-designed for lubrication.

Group V: All other base oils not classified in Groups I–IV. Includes esters, polyalkylene glycols, silicones, and others. Often used as co-base stocks in premium formulations.

Why the base oil group matters: the molecular difference

Mineral oil (Groups I and II) is processed from crude petroleum. Crude contains a vast mixture of hydrocarbon molecules of different sizes and structures — including aromatics, naphthenes, sulphur compounds, and waxes. Refining removes some of these, but a mineral base oil still contains molecules of many different shapes and weights. The molecular weight distribution is broad.

PAO (Group IV) is chemically synthesised. Small ethylene molecules are polymerised under controlled conditions to produce large, uniform molecules. Every molecule is the same size, the same shape, the same structure. There are no aromatic compounds, no waxes, no sulphur contaminants.

This structural difference has direct performance consequences. Because mineral oil molecules vary in size and weight, they respond differently to temperature — some thin faster than others, producing unpredictable viscosity behaviour under real engine conditions. PAO molecules respond predictably and uniformly. This is why PAO achieves higher Viscosity Index, better oxidation stability, and higher inherent shear resistance without relying as heavily on additive chemistry to compensate for molecular variability.

Viscosity Index and what it reveals

Viscosity Index measures how much an oil's viscosity changes as temperature rises. A higher VI means the oil holds its grade more consistently between cold and operating temperatures:

Group I/II mineral oils: VI typically 80–120
Group III (the "synthetic" mineral oil): VI ≥120, typically 125–140
Group IV PAO: VI typically 140–175

Two oils both labelled "fully synthetic 10W-40" can have VI values of 130 and 160. At 100°C (the standard test temperature), both pass. At 110°C — the sustained oil temperature in a small motorcycle engine during slow urban traffic — the higher-VI oil has meaningfully more viscosity remaining. In SEA conditions, where oil temperatures are sustained at the upper end of the operating range, this is not theoretical. It matters at every traffic light.

The HTHS viscosity test (high-temperature high-shear, measured at 150°C under 10⁶ s¹ shear) captures this even more directly — it simulates the conditions inside a bearing under load, at operating temperature. A PAO-based oil will typically hold a higher HTHS value than a Group III oil at the same nominal grade.

What this means for JASO ratings and additive packages

The base oil group affects performance, but the additive package determines JASO compatibility. Group III "synthetics" can carry JASO MA2 ratings. The base oil group alone does not determine whether an oil is safe for wet-clutch use. The friction modifier content — which the JASO test specifically measures — is what matters for clutch compatibility.

A Group III "synthetic" with a well-formulated additive package, correctly rated JASO MA2, will protect a wet-clutch motorcycle correctly. A Group IV PAO oil with an incorrect additive package may not. Both the base oil and the additive package are part of the answer.

Key takeaways