Lube Notes: Petroleum Oil vs Synthetic Oil - Oxidation Stability

Volume 2 Issue 1 - Lubrication

Article Index
Lube Notes: Petroleum Oil vs Synthetic Oil
Oxidation Stability
Lubrication/Wear Protection
Petroleum Oil vs Synthetic Oil: Cost
All Pages

Oxidation Stability

How well does the oil resist oxidation and sludge formation? As oil oxidizes, it thickens (viscosity increases) and deposits sludge in the engine. Sludge may eventually clog critical oil passages, preventing necessary oil from reaching vital engine parts. This causes excessive wear and, eventually, failure of various engine parts. Synthetics are inert, meaning there are no polar sites (having positive or negative polarity) and simply do not react with oxygen. Petroleum oils are highly polar (mostly positive polarity) and readily react with oxygen. To counter this reactivity, petroleum oils are treated with anti-oxidation additives. When oils are operating in the intermittent range (temporarily outside the normal operating band) they are susceptible to higher rates of oxidation. Unfortunately, today’s engines are forcing oils to operate routinely at 230°F to 250°F. This puts the petroleum oils in a range of temperatures that causes increased use of the anti-oxidants in the additive package and shortens the life of the oil. PAO or Ester based synthetics are in the normal operating band for temperatures in excess of 330°F and suffer little or no oxidation. This is one of the reasons you hear of mechanics reporting how clean engines with synthetics are, even those with high mileage.

> Oxidation Stability Advantage: Very Strong for Synthetics


How easily does the oil vaporize or boil off? When oils are hot, vaporization can result in significant oil consumption and thickening of the oil. Not only is this a problem for oil consumption, but the oil vapor is sucked into the engine via the Positive Crankcase Ventilation system, contributing to significant hydrocarbons in the exhaust (PCV systems have been used in gasoline engines and are now starting to be used in diesel engines). In petroleum oils, the molecular structure is non-uniform, consisting of various size compounds. Imagine countless footballs, baseballs, hockey sticks and tennis rackets all mixed together, pushing against each other. When the oil gets hot, some of the lightweight items are liberated and fly away while the larger, heavier items remain. As this process continues, only the larger items remain, resulting in much thicker oil. In contrast, the molecular structure of synthetics is like a bunch of identical golf balls, all the same size and tightly packed together, resisting vaporization; as a result, they stay in grade for much longer periods and reduce oil consumption.

>Volatility Advantage: Strong for Synthetics



How does the oil affect the seals? Will it cause them to shrink or to swell? And, is the oil chemically compatible with them? Seals are made of a variety of compounds in order to provide rigid but flexible surfaces that promote good sealing in order to keep liquids in and dirt out. Petroleum oils are fully compatible with the seal materials used in modern engines and will slightly swell the seals. While PAO synthetics tend to shrink seals, Esters tend to swell the seals: both are chemically compatible. In synthetics where PAO is the primary base oil, another synthetic oil, Diester for example, is used to provide the desired seal swell and nourishment for seals. Historically, seal compatibility issues have caused real and imaginary problems for synthetic oils in the market place. Currently, seal issues for properly blended synthetic oils are no longer an issue.

> Seals Advantage: Slight for Petroleum


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