1. Synthetic Oils

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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