Base oils and the quiet science of a good lubricant

Pour a litre of modern engine oil and roughly nine-tenths of it is base oil, the clear carrier fluid that does the actual lubricating. The additive package gets the marketing, but the base oil sets the floor for how the finished product behaves in a cold engine at start-up and a hot one at full load.

What a base oil does

A lubricant has one core job: keep two metal surfaces apart with a thin film of fluid. The base oil is that film. It has to stay thick enough to carry load when the engine is hot, thin enough to flow when the engine is cold, resist oxidation over thousands of hours, and not boil off into the exhaust. Everything else, the detergents, the anti-wear agents, the viscosity modifiers, is blended on top. Get the base oil wrong and no additive package can rescue it.

Base oils come from the heavy end of the barrel, the vacuum gas oil that distillation leaves behind. How that feedstock is treated decides which group the finished base oil belongs to, and the group is what sets the price.

The Group I to III ladder

The American Petroleum Institute sorts mineral base oils into groups by three measures: saturates, sulphur, and viscosity index. Moving up the ladder means a cleaner, more stable oil that holds its viscosity across a wider temperature range.

• Group I. Solvent-refined, the original process. Under 90 percent saturates and higher sulphur, with a viscosity index of 80 to 120. Cheapest to make and slowly being retired.
• Group II. Hydrotreated to above 90 percent saturates and very low sulphur. Clearer, more stable, and now the workhorse of the market.
• Group III. Severely hydrocracked to a viscosity index above 120. Clean and stable enough to be sold as synthetic in many finished oils.
• Groups IV and V. True synthetics such as polyalphaolefins and esters, blended in where extreme performance justifies the cost.

The market is climbing that ladder. Tighter engine-oil specifications, which demand low volatility and better fuel economy, are steadily pulling volume out of Group I and into Groups II and III. Refineries that only make Group I are closing one by one.

The additive package gets the marketing, but the base oil decides whether the film holds.

Reading a base-oil spec

When a blender buys base oil, they read a short list of numbers and match them to the finished product they are building.

• Kinematic viscosity at 40 and 100 degrees Celsius sets the grade, an SN150 or a 500N.
• Viscosity index shows how much that viscosity changes with temperature; a higher number means a flatter response and a better multigrade oil.
• Pour point is the lowest temperature at which the oil still flows, which decides a cold winter start.
• Volatility, usually the Noack test, measures how much oil boils off at heat, which drives consumption and emissions.

A 5W-30 built to protect a turbocharged engine for 20,000 kilometres needs a high-index, low-volatility Group III base; a simple hydraulic fluid sits happily on Group I or II. We supply across the ladder, by group and viscosity grade, on FOB and CIF terms with a certificate of analysis at the load port.