Sulphur in oil - impurity, oil component or additive?

On my OELCHECK lab report, the sulfur content is listed under the "Additives" section. This usually relatively high value catches the eye immediately.
What does the high sulfur content mean in the context of my lubricant?

Table of contents

  1. Sulfur at OELCHECK
  2. Not all sulfur is the same
  3. Sulfur from the base oil
  4. Sulfur as an additive
  5. Sulfur as a contaminant

Sulfur at OELCHECK

OELCHECK determines more than 30 elements with an AES device from PerkinElmer, which works according to the ICP principle. This is used to determine additive, contamination and wear metals, some of which are only indicated in the report if they have been detected.

The specification of sulfur can be found on almost all OELCHECK laboratory reports in the additives section, thus complementing existing elements such as calcium, magnesium, boron, zinc, phosphorus, molybdenum and barium.

Sulfur is often the highest value.

Not all sulfur is the same

There may be various causes for the presence of sulfur in the oil. In mineral oils, sulfur is usually a component of the base oil, but sulfur can also be added to the oil as an anti-wear additive. And in some cases, it represents contamination.

Sulfur from the base oil

The starting product for most lubricants and fuels is crude oil. The crude oil consists largely of hydrocarbons. When it is extracted, it is heavily contaminated with water, sulfur, nitrogen, oxygen, and with solid foreign matter or metallic elements. Depending on which part of the world the black gold comes from, the sulfur content can be less than 1% (10,000 mg/kg or ppm) or up to about 6% (60,000 ppm).

The removal of sulfur from crude oil is very costly in the refinery; consequently, finished products can be extracted from low-sulfur oil at lower cost. Thus, the sulfur content also affects the price of crude oil. Sulfur is bound in crude oil in various compound forms, such as hydrogen sulfide (H2S).

Too much sulfur in the base oil of lubricants can cause acid formation, among other things, which causes non-ferrous metal corrosion. To avoid this, the starting product must be desulfurized. In Germany, the maximum permissible sulfur content of some petroleum products, especially fuels, is prescribed by law.

For example, to reduce pollutant emissions, diesel fuel must not contain more than 10 ppm or 0.001% sulfur. Thanks to this regulation, environmental pollution from acid rain containing sulfuric acid has decreased dramatically.

However, the level of sulfur in the base oil used to produce an engine, gear or hydraulic oil depends on its geographic origin, the degree of desulfurization and, ultimately, the price paid for it. Even oils from the same producer and with identical product names can show significant base oil-related differences in sulfur content, especially if the oils are produced in different countries

Sulfur as an additive

To prevent wear and reduce friction, sulfur-containing EP (extreme pressure), HD (heavy-duty) and AW (anti-wear) additives are added to many oils. These are very polar and accumulate on the metal surface. Under load, a chemical reaction occurs between the metallic asperity peaks of the surfaces with the additive components consisting of sulfur and phosphorus compounds.

This forms a reaction layer that reduces wear and friction. The material in the reaction layer is slightly softer, so that breaking or local welding of the asperity peaks is avoided and the risk of "galling" is reduced. The surface roughness changes and the contact area of the surface is increased.

In the laboratory, however, it is not possible to determine how much "active" sulfur, i.e. sulfur added by additives and thus having a positive effect, is present in an oil sample or how much "bound" sulfur originates from the base oil. Even oils with the same viscosity cannot be inferred to have the same initial sulfur value. The amount of EP/AW additive needed for a functioning oil, and therefore the total sulfur content in the oil, depends on the oil type and application requirements. In the case of oils for hypoid gears, wear protection on the tooth flanks under difficult mixed friction conditions is essential..

The sulfur content can be correspondingly high (up to 3% or 30,000 mg/kg). The values in the table below are for guidance only. Depending on the additive philosophy of the oil manufacturer and the desulfurization of the base oil, considerable deviations may occur. Since synthetic base oils are produced from technical gases without sulfur content, the sulfur content in bio-oils or other synthetic lubricants is usually only due to additives and is therefore significantly lower than in mineral oils.

Typical sulfur values
Engine oils 5,000 - 18,000 ppm
Hydraulic oils 1,000 - 6,000 ppm
Hydraulic oils (zinc-free) 1,000 - 15,000 ppm
Synthetic hydraulic oils 100 - 3,000 ppm
Oils for industrial gears 3,000 - 20,000 ppm
Hypoid gear oils 5,000 – 30,000 ppm
Turbine oils (without EP) < 10 - 300 ppm
Circulating lubrication 1,000 - 10,000 ppm
Diesel fuel < 10 ppm
Low-sulfur fuel oil < 50 ppm
Gasoline < 10 ppm
Biodiesel, rapeseed oil < 10 ppm

Sulfur as a contaminant

In most applications, the sulfur content will change little during the service life of the oil. The amount depends essentially on the initial value for the fresh oil. Since fuels today are only allowed to contain a small amount of sulfur, hardly any fuel-related changes in sulfur can be detected, even in used engine oils with a high fuel input.

Gas engines are increasingly powered by biogas. Hydrogen sulfide is a gas that is often produced during putrefaction processes due to the decomposition of proteins. When gas engines are operated with biogas or sewage gas, the sulfur originating from the gas can also enter the gas engine oil in the form of sulfurous acids and accumulate here in a harmful manner. In these cases, trend monitoring of the sulfur content can provide indications of necessary oil changes due to fluctuating gas qualities, for example.

The level of sulfur content in an oil depends almost exclusively on the base oil and additives. As with all other additive elements, however, it is very difficult to make a qualitative assessment of an oil on the basis of the sulfur content. Elemental analysis alone does not give a clear indication of whether the sulfur comes from the base oil or whether it has been added to the oil as an additive component. But if your tank smells like rotten eggs, that's usually an indication that the sulfur was added as an additive. The unpleasant smell does not occur because the oil has gone bad. Rather, the sulfur was so heavily demanded as wear protection that its chemical form changed. A complete oil analysis with the observation of the sulfur content in comparison to the fresh oil shows whether it is still sufficiently effective in this case and whether the oil can still be used.