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Base Oils explained
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.
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.
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 - see Viscosity explained) 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.
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
-
-
-
-
-
Cleaner
engine surfaces
-
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.
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.