Petroleum Oil vs Synthetic Oil, Base Stock and Additives, Amsoil Synthetic Oil

Lube Notes: Petroleum and Synthetic Oil Base Stocks and Additives

Volume 1 Issue 4 - Lubrication

Article Index
Lube Notes: Petroleum and Synthetic Oil Base Stocks and Additives
Selecting Additives
All Pages

Lube NotesIn the preceding Lube Notes, we covered basic lubrication, oil functions, additives and base stocks. Now, it’s time to construct finished lubricating oils. From what we have learned, it may seem like the only thing we need to do is pick a base stock oil, mix in some additives and presto, we have lubricating oil. If only it was that simple. Of course, it’s not.

Previously, we looked at the refining of petroleum and classifications for synthetic and petroleum oils. The base stock with which we choose to start will obviously have a direct bearing on the quality of the finished product. If cost is no concern, then all finished oils would be made using one of the synthetic base oils since they result in the best lubricating oils. However, cost is an important factor and will always be a consideration in choosing base stock oils. Most oils are manufactured by a reverse process where the final performance requirements dictate the quality or lack of quality of the ingredients. If the manufacturer is making an oil to meet the minimum performance criteria for the current classification, then no money will be spent on anything more than an adequate base stock. On the other hand, if the manufacturer is producing a high performance oil, then he will spend what is reasonably necessary to produce the final product’s higher level of performance.

Selecting a Base Stock

Considering that 95% of modern motor oils are multi-grade oils, the base stock oil will be a determining factor in the amount of Viscosity Improvers (VI) added to achieve the grade range of the finished oil. The lower the quality of the base stock, the more viscosity improver required in order to make the oil multi-grade (i.e., 5W-30, 10W-40, etc). VI additives are long chain, high molecular weight polymers that may serve some additional functions such as pour point depressants or dispersants. They are expensive and under extreme stress may suffer mechanical sheer. In most cases it makes sense to use better quality base stock oils in order to use less viscosity improver additives. Some synthetic base stock oils (Poly Alpha Olefins – PAOs and Esters) have such high natural Viscosity Indices that little if any additives are used to achieve the multi-grade finished product. These synthetics can pass the 5W viscosity test (winter) and the 30 viscosity (operational viscosity at 210*F) without the aid of viscosity improver additives.


Today, most petroleum base stock oils come from Group II (see table A). Group II oils are a definite improvement over Group I oils and current petroleum based oils are much better than those of 20 years ago. Synthetics are a little more varied than petroleum base stocks and are split between Group III hydro-cracked petroleum (addressed in the Summer ‘08 issue), PAO’s and Esters. Today, most synthetics for motor oils are made using Group III base stocks. Mobil and Amsoil continue to use Group IV PAOs (Amsoil blends Ester with PAO) with Redline oils coming primarily from Group V Esters.

Petroleum Oil vs Synthetic Oil Chart

Not all oils are created equally. The above chart shows the performance of seven motor oils in two catagories: oil evaporation expressed in percentage and pour point expressed in degrees fahrenheit. The two better performers (green) relative to the others are synthetics; the second from the left, the poorest performer listed, is a synthetic blend – as is the oil furthest to the right.


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