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LO 10-4 Describe the composition of synthetic oil and outline the advantages and disadvantages of synthetic oil

Site: CDX Learning Systems
Course: DE Section 2-1: Engine Blocks and Cylinder Heads
Book: LO 10-4 Describe the composition of synthetic oil and outline the advantages and disadvantages of synthetic oil
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Date: Thursday, 5 November 2026, 7:49 PM

Table of contents

1. Synthetic Oils

LO 10-04: Describe the composition of synthetic oil and outline the advantages and disadvantages of synthetic oil.

 

Synthetic Oils

Synthetic-base lubricants have been used in jet engines and special-purpose engines since World War II. Tanks used in World War II needed synthetics to allow the oil to properly lubricate the engines in extreme cold while jet engines needed synthetics to handle high operating temperatures that would burn petroleum-based oils. Synthetic lubricants did not begin to make in-roads into automotive applications, however, until the late 1970s.

The term synthetic oils generally refers to oils with a synthetically derived or manufactured base stock. While some brands of synthetic oils are derived from ultra-refined petroleum oil, the most popular base stock for synthetic oil is created from a petroleum-derived molecule called polyalphaolefin (PAO; TABLE 10-3) . PAO molecules are smaller and more consistent in size, and no impurities are found in this oil because it is derived through a chemical process, making it chemically stable and flexible compared to conventional oil Figure 24. Approximately 70% of the base stock of synthetic oils is made from PAO. Chemical additives are then blended with the synthetic oil base stock, similar to the process of making conventional oils. However, synthetic oils usually have more additives than conventional oils. No API or SAE standards exist yet for defining what synthetic oil is; however, they do need to meet minimum API and SAE criteria if they carry the API symbols.

   Fig10_24

FIGURE 10-24: Comparing the size of conventional mineral oil and synthetic oil molecules.

 

  TABLE 10-3: Sources of Base Stock Lubrication Oil  
Oil Base Stock Disadvantages Column 3
Conventional mineral oil Mineral oil derived from crude oil Contains undesirable impurities, such as oxygen, sulfur, nitrogen compounds, trace metals, and carbon residues. Additive package cannot operate effectively because the additive has to compete with the impurities when bonding with base oil molecules. These oils break down under extreme heat and form solids under extreme cold.
Hydro-processed mineral oil Highly refined mineral oil Contains some undesirable chemical impurities but is more refined than conventional mineral oil.
Severe hydro-processed mineral oil (considered synthetic in North America only) Ultra-refined mineral oil (may be also blended with PAO to make semi-synthetic oils) Requires additional processing and removal of more impurities than hydro-processed mineral oil, but some impurities still exist.
Fully synthetic oil (chemically derived) Chemically manufactured (PAO) Expensive

 

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1.1. Benefits of Synthetic Oil

LO 10-04: Describe the composition of synthetic oil and outline the advantages and disadvantages of synthetic oil.

 

Benefits of Synthetic Oil

Manufacturers claim the following benefits for synthetic oils:

  • Lower coefficient of friction (slipperier): Users of synthetic oil have claimed increases in their fuel mileage ranging from 2–5%.
  • Improved low-temperature viscosities: Conventional oils tend to include wax molecules that thicken oil at lower temperatures. Typical synthetic 15W-40 oil remains a liquid and flows even at −58° F (−50° C).
  • Improved high-temperature performance: Synthetic oils have few light or thin hydrocarbon molecules, which tend to evaporate at high temperatures.
  • Greater resistance to chemical breakdown, oxidation, coking, and deposit formation.
  • Decreased oil consumption: When synthetic oil is used, 42% less oil consumption occurs. This is due to less evaporation of the thin ends of hot oil and resistance to converting to deposit formation.
  • Fewer chemical impurities: This means additives do not adversely react with impurities, prolonging the usefulness of the additive package.
  • Reduced friction and engine wear because of a lower coefficient of friction.
  • Longer intervals between oil changes due to higher resistance to oxidation TABLE 10-4.
  TABLE 10-4: A Comparison Between Two Leading Brands of Oil  
Measurement of Oil Performance Synthetic 10W-40 (PAO Base Stock) Petroleum 10W-40 (Poor Quality)
Effective lubrication range –60 to 400° F (–51 to 204° C) 0–300º F (–18 to 149° C)
Viscosity increase after 64-hour running test >9% >102–400%
Volatility (evaporation at 300° F [149° C] after 22 hours) 1% 28%
Crankcase temperature (track test) 240° F (116° C) 290° F (143° C)
Flash point (D92 test) 470° F (243° C) 400° F (204° C)
Film strength 3000 psi (207 bar) routinely observed 500–2000 psi (345–138 bar)

 

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1.2. Disadvantages of Synthetic Oil

LO 10-04: Describe the composition of synthetic oil and outline the advantages and disadvantages of synthetic oil.

 

Disadvantages of Synthetic Oil
The primary disadvantage of synthetic oils is that they cost significantly more than mineral oils. Synthetic oil manufacturers, however, suggest that this problem is offset by long service life and improved performance. Manufacturers of almost every LD and most MD diesel engines recommend synthetics because less wear is likely during cold start-ups because oil will reach turbocharger and camshaft bearings faster than conventional oil. Synthetic PAO-based oil is also less likely to deteriorate during high-speed, high-load operation in smaller displacement diesels.

Another problem with synthetics is that the molecules are more uniform in size and thus are more likely to leak or weep through worn seals and lose gaskets. This explains why more engine oil leaks are observed when changing over to synthetic oil after using conventional engine oil.

Because synthetic oils have a lower coefficient of friction, they are not recommended for break-in periods. The best application for synthetics is in extreme operating conditions, such as heat and cold. Equipment with high initial cost and long chassis life can also benefit from the extended engine longevity that can result from using synthetics. In other words, synthetics protect high-value investments through longer engine life.

  Technicians Tip  
  Synthetic oils are of tremendous benefit to high-speed, turbocharged diesels for several reasons:
  • Turbochargers and rocker shafts receive lubrication more quickly in comparison to petroleum-based oils. Because synthetics flow easily at cold temperatures, engine components are not starved for oil.
  • Synthetic oils are resistant to coking. During hot shutdown of turbocharged engines, exhaust heat in the turbine housing moves into the center bearing housing, which can cause oil on turbine shaft bearings to burn and coke. The abrasive carbon residue from coked oil can damage shaft bearings and score the turbine shaft.
  • More flexible molecules provide improved shear strength. Engines using unit injectors exert greater stress on oil through shearing, causing oil to thin out. The smaller synthetic oil molecule is less affected by shear and enables viscosity to remain stable over a longer period of time.
  

 

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