Base Oils Explained

In this article...

  • Mineral, Semi-Synthetic or Synthetic base stocks
  • What are the advantages of Synthetics over Mineral oils?
  • What are the downsides to Synthetics?
  • What about Semi-Synthetics?
  • Mineral versus Synthetic
  • What does this API Group rating mean to the user?
  • Will I find this Group Rating easily?
  • Can a Mineral Oil be called Synthetic?

Mineral, Semi-Synthetic or Synthetic base stocks

Base stocks are either mineral based, semi-synthetic, synthetic or vegetable based.  Most motor oils were mineral based until the late 1990s when synthetics became more widely available at more affordable prices and now tend to dominate the passenger car motor oil market. On an industrial scale mineral oils still predominate with syntehtics being used for exceptional cases that demand the use of a synthetic base oil.   

Mineral oils now fall into three main blends as categorised in Table 1 below.  Improvements in blending of the base oils has reduced some of the problems that were typical of oils in the 1960s and 1970s.  Depending on the level of refining, mineral oils can still suffer from inconsistent molecular sizing, weakness of unsaturated bonds and impurities such as Sulphur and Aromatics leading to shorter oil service life, poor film strength, low Viscosity Indices, and depositing on machine surfaces.  Again, though, price will determine the quality of the base stock used in mineral based oils.

Synthetics are derived by a different refining process to offer better performance owing to their consistent molecular structure and purity.  

Figure 4 - Graphical representation of the difference in molecular sizing between mineral and synthetic base oils.

What are the advantages of Synthetics over Mineral oils?

  • Superior wear control
  • Superior friction control
  • Superior thermal stability
  • Superior aging characteristics
  • Superior film strength
  • Higher Viscosity Indices
  • Superior detergency levels

 Translated, that means:

  • Improved fuel economy or energy utilization
  • Improved power output
  • Cleaner machine surfaces
  • Longer oil life with longer oil change intervals resulting in less downtime
  • Reduced component wear

What are the downsides to Synthetics?

Basically, synthetic oils cost as much as 3 times the cost of mineral derived oils. In real terms that is the only downside bar potential seal and coating compatability issues.   Other issues often given as negatives of synthetics such as seal compatibility and additive solvency can be controlled.  Issues such as viscosity being too low or thin are often quoted as a reason for not being suitable often on forums relating to use in a classic car will depend on the selection of the correct oil viscosity grade.  Unfortunately in the classic car community synthetics receive bad press for all the wrong reasons.

What about Semi-Synthetics?

The implication is superior performance at a lower cost by combining a mineral and a synthetic base oil stock.  There are no regulatory controls on what percentage mix constitutes a semi, so price and performance variation will occur.  Do not be mislead into thinking that the price difference on semi-synthetics is simply a marketing ploy, although it may well be, but so-called semi-synthetics can vary between 2% and 30% typically in terms of synthetic base oil content blended with the mineral base oil.

Mineral versus Synthetic

The table below identifies the American Petroleum Institute (API) grading of base oils. Group I, II and III are all derived from crude oil which in effect means these are a mineral base oil. Group 4 is as close as you can get to a mineral oil in nature owing to it's derivation from the Olefins in the gas industry.  Group V oils will include all other forms of synthetically engineered oils such as Glycol and Ester based fluids, as well as Silicone fluids.


 Table 1 – API Base Oil Categories

What does this API Group rating mean to the user?

  • The higher the number of saturates, the higher molecular bond strength of the oil and therefore the better the resistance to breakdown or loss of viscosity. 

  • The lower the Sulphur content, the better the purity and thus the less the corrosive and oxidation potential that exists.

  • Viscosity Index (VI) is an indication of the rate of change of viscosity against temperature, and the higher the VI, the better.

Will I find this Group Rating easily?

No, generally manufacturers will refer to either Mineral, Semi-synthetic or Synthetic, but won’t differentiate as to the Group on the information on the oil container.  If you dig deep enough to find the data sheet you may find the answer and this is crucial when dealing particularly with turbine oils where these are used in large volumes.

Can a Mineral Oil be called Synthetic?

The answer is yes.  With the advances in refining of crude oils, a process of hydrogen cracking is used to ensure low levels of Sulphur, Aromatics and improved levels of saturated bonds.  The argument that was put forward and won in the North American market is that this type of mineral base oil is effectively similar to a synthetic oil in performance terms so in effect the marketing department can legally use the term synthetic (a very emotive term) for Group III base oils.  Please note that this really only applies in the North American market and not elsewhere.