Lube Notes 5 - Synthetic Versus Petroleum

The ongoing march to achieve more technologically advanced engines continues and certainly the GM Duramax diesel engine exemplifies that quest. The race between GM and its would-be competition has benefited you and me: the improvement in all aspects of these diesel engines is easily quantifiable in terms of horsepower and torque as well as fuel efficiency and endurance. Recognizing how vastly improved these diesels are to their predecessors, it should not surprise anyone that advances in the lubricants for these engines have also facilitated quantum leaps in performance.

Any oil, properly rated for use in a high performance turbo-charged engine, is a remarkable lubricant regardless of the base oil used. In this article, I will compare synthetic diesel engine oil to petroleum diesel engine oil and draw some conclusions and make some recommendations. Previous Lube Notes have established fundamentals of lubrication and how oil is made, so if you haven’t read those, a review might be in order. I am writing this article assuming you have read the preceding articles.

To start, we should compare several performance criteria for petroleum oil vs synthetic oil...

Thermal Stability

How well does the oil hold viscosity as temperature increases? This is reflected in the Viscosity Index (VI) rating, with a higher number indicating greater stability. Petroleum oils rarely exceed 100 on the Viscosity Index while some synthetics rate higher than 180. Oils that maintain rated viscosity – instead of thinning out at higher temperatures – perform better in your engine. Thin oil will reduce film strength and result in higher wear rates of critical engine parts.

> Thermal Stability Advantage: Strong for Synthetic Oil

Higher viscosity index (VI) liquids are less responsive to temperature extremes. At 0°F, the VI 95 petroleum oil is thicker (measured in centistokes, a dynamic measure of resistance-to-flow) than the synthetic oil with a VI of 150. On the hot side at 210°F, the VI 150 synthetic maintains viscosity better than the VI 95 petroleum oil that thins out more easily.

Temperature Range

What are the highest and lowest temperatures the oil can tolerate and still provide proper lubrication, during continuous or intermittent duty? This range is established by measuring the pour point (lowest temperature the oil will pour) and the highest temperature at which the oil can hold sufficient viscosity in order to provide lubrication. Chart Two demonstrates the superior performance of synthetic oil vs petroleum oil. Group III hydro-cracked synthetics are not on the graph and it is important to note that the Group III synthetics will have similar cold flow performance but significantly less high temperature performance compared to Polyalphaolefins (PAO). PAO and Dibasic Acid Esters are the primary chemicals used in engine and drive line oils. A strong temperature range is paramount to providing proper lubrication, especially in severe duty or extreme temperatures.

> Temperature Range Advantage: Very Strong for Synthetic Oil

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

Volatility

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

Seals

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