Mineral Oils

What are mineral oils?

Mineral oils are traditional lubricants that are products of crude oil refining. For decades, they have been the foundation of most engine, gear, hydraulic, and industrial oils. Although they are increasingly being replaced by more advanced synthetic oils, they are still widely used in many applications.

How are mineral oils produced?

Mineral oils are obtained by distilling and refining crude oil. This process aims to separate the desired hydrocarbon fractions with the appropriate viscosity and remove unwanted impurities such as sulfur, aromatic compounds, or paraffins.

Depending on the degree and type of processing, base mineral oils are classified according to API (American Petroleum Institute) into:

• Group I: These are the least processed base oils obtained by solvent methods. They are characterized by a lower viscosity index and a higher sulfur and unsaturated compound content. Used in less demanding applications or where price is a key factor.
• Group II: Base oils subjected to mild hydrocracking. They have better properties than Group I, such as a higher viscosity index, better thermal and oxidative stability, and lower impurity content. They are the basis of many popular mineral oils.

The finished mineral oil is a mixture of base oil (or several base oils) and a properly selected package of additives that enhance its properties (e.g., anti-wear, anti-corrosive, detergents, dispersants, viscosity modifiers).

Basic properties and characteristics

Mineral oils are characterized by:

• Complex molecular structure: They contain a mixture of different types of hydrocarbons with varied structures and molecule sizes.
• Presence of natural impurities: Despite refining processes, they always contain certain amounts of sulfur, nitrogen, or aromatic compounds.
• Satisfactory lubricating properties: Provide an adequate lubrication film under typical working conditions.
• Lower cost: Compared to synthetic oils.

Comparison with synthetic oils – pros and cons

Advantages of mineral oils:

• Lower cost: They are usually cheaper to produce and purchase.
• Good compatibility with seals: In older constructions, where traditional sealing materials were used, mineral oils may be a safer choice.
• Proven in many applications: They have a long history of use.

Disadvantages of mineral oils (compared to synthetic):

• Lower thermal and oxidative stability: Degrade faster at high temperatures, leading to deposits and reduced oil life.
• Poorer performance in low temperatures: Thicken in low temperatures, making starting difficult and delaying oil arrival to lubrication points.
• Lower viscosity index (VI): Their viscosity changes more with temperature.
• Higher volatility: Greater oil loss through evaporation.
• Need for more frequent changes: Due to faster aging.

Typical applications

Despite the growing popularity of synthetics, mineral oils are still effectively used in many areas:

• Older vehicle models: Where there aren’t high demands on oil parameters.
• Agricultural and construction machinery: Working in moderate conditions.
• Some hydraulic systems and industrial transmissions: Where operating conditions are not extreme.
• Applications where the low cost of the lubricant is a key criterion.
• As break-in oils for new engines (in some cases).

When is the choice of mineral oil justified?

The choice of mineral oil can be justified when:

• The equipment or vehicle manufacturer permits or recommends the use of a mineral oil with a specific specification.
• The working conditions are not extreme (moderate temperatures, loads).
• The intervals between changes are relatively short.
• The budget for lubricants is limited, and mineral oil meets the minimum technical requirements.

Always follow the equipment manufacturer’s (OEM) recommendations in the operating manual. Improper oil selection, even if it’s a high-quality synthetic where it is not needed or even not advisable (e.g., in some very old constructions), may not bring expected benefits or could even be harmful.