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Some Myths about Engine Oils

Oil pressure is caused by the resistance of the oil to flow (viscosity) under the pumping action of the oil pump.  With wide oil galleries and low viscosity oil, flow would be rapid and oil pressure low - a very desirable condition for minimizing wear.  Conversely, under the same pumping conditions; with narrow oil galleries (by design or by blockage) and high viscosity oil, the oil flow will be slow & oil pressure will be high, hence resulting in less efficient lubrication.

» "Good" oil will be one that has viscosity characteristics sufficient to give good hydrodynamic lubrication of the loaded surfaces, yet flows around the engine well to provide a continuous supply of fresh lubricant.
 
» In an extreme case, very viscous oil would never get to the critical parts to do its lubrication job. Note in this case, that engines do have by-pass valves to prevent excessive oil pressure in case of filter blockage
 
 

Oil Pressure Gauge Interpretation

» Under ideal circumstances, oil pressure should be stable and any large increase or decrease should be investigated for possible mechanical damage.
 
 
Any deviation from what the driver considers normal oil pressure usually results in the engine oil being blamed.  Driver awareness of this situation increases when oil pressure gauges are used instead of an oil indicator light, which only illuminates at low oil pressure.  

Influence of Engine Age
As an engine ages, the gradual increase in engine clearances as a result of normal wear will produce a drop in oil pressure. A high mileage engine will usually register lower oil pressure than a new or rebuilt one.

Influence of Viscosity
The only property of engine oil that can affect engine oil pressure is the oil’s viscosity, and oil viscosity is temperature related.  However, engine oil viscosity has less effect on oil pressure when the engine is in good mechanical conditions.  At normal driving speeds, there should be little difference in oil pressure between an SAE 10W 30 and SAE 20W 50 engine oil if the engine is in good mechanical condition.  If engine oil pressure responds to a change in SAE grades, this is usually an indication that the engine is in need of mechanical attention.

Influence of Temperature
It is possible to have variations in oil pressure if temperature extremes are encountered. Very low oil temperatures result in high oil pressures such as when an engine is first started during cold weather.  High oil temperatures, which would cause the engine oil to thin out result in lower oil pressure.

Remember that the registered oil pressure is only an indication of the proper functioning of the oiling system. Sudden changes or wide variations in oil pressure should be considered as a warning that the engine is in need of attention. Abnormal oil pressures are seldom the fault of the engine oil.

The following is a list of some of the possible reasons for abnormal oil pressure.