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.
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 engine 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?
Translated, that means:
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. Other issues often given as negatives of synthetics such as seal compatibility and additive solvency can be controlled. Issues such as viscosity often quoted as a reason for not being suitable in a classic car will depend on the selection of the correct oil. 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.
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?
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.
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.