Mineral vs Synthetic Oil: What You're Really Deciding When You Choose

What each means at the molecular level

Mineral oil is obtained by refining crude petroleum. It contains a mixture of hydrocarbon chains of variable length, including paraffins, naphthenes, and aromatic hydrocarbons depending on the degree of refining. The hydrocracking (HC) process produces high-quality mineral oils — sometimes called 'semi-synthetic' — with lower aromatic content and viscosity index superior to conventional mineral.

Synthetic oil (primarily PAO, polyalphaolefin, and synthetic esters) is manufactured by chemical synthesis from controlled monomers. The result is a structurally uniform molecule, with a narrow molecular weight distribution and without the problematic compounds of crude petroleum.

The viscosity index: why it matters more than it seems

The viscosity index (VI) measures how much a lubricant's viscosity changes with temperature. A conventional mineral oil has a VI of 90–100. A PAO oil has a VI of 140–160. A synthetic ester can exceed 180.

In practice, this means that a synthetic oil better maintains its lubricating capacity both in cold (starts) and hot (maximum operating temperature) conditions. For applications with wide temperature ranges — outdoor reducers in extreme climates, refrigeration compressors — the synthetic's VI advantage is decisive.

Compatibility with seals and elastomers

This is the factor that generates the most surprises. PAO synthetic oils are compatible with most elastomers, but not all. Synthetic esters can cause slight swelling in NBR and FKM, which in many cases is acceptable or even desirable (improves sealing), but in others can cause seal damage.

Before changing from mineral to synthetic oil in an existing system, compatibility with the current seals must be verified. In new systems, the design can account for the oil type from the outset.

Drain interval: the strongest argument for synthetic

A high-quality conventional mineral oil in an industrial reducer has a typical drain interval of 4,000–6,000 hours. An equivalent quality PAO in the same application can reach 8,000–12,000 hours. In some specific applications with good analytical monitoring, 20,000 hours.

The extended interval has a direct impact on maintenance costs: fewer oil changes = fewer planned stoppages, less waste oil, less labour. These savings are easily quantified and typically exceed the extra cost of synthetic oil in less than a year.

When synthetic is mandatory

• Continuous operating temperature above 80°C for hydraulic oils or above 100°C for gear oils.
• Start-up temperatures below -20°C where conventional mineral loses pumpability.
• High-speed applications where the viscosity index directly influences energy consumption.
• H1 food-grade lubricants: only available in synthetic base (PAO, ester, or white oil).
• When the equipment manufacturer explicitly requires it in their specifications.

When mineral is the right choice

• Low-pressure hydraulic systems in controlled ambient temperature with short drain intervals.
• High-volume applications with tight budgets where extended service life does not justify the price differential.
• Equipment in which existing seals are not compatible with synthetic and the replacement cost is not justified.
• When in-service oil analysis confirms that the mineral is within specification and there is no accelerated degradation.