Choosing the best motor oil for turbocharged engines.

With ever tightening emissions regulations and demands to reduce fuel consumption smaller turbocharged engines are becoming the norm. Although these engines are very efficient the turbocharger generates very high thermal loads that the motor oil has to cope with, requiring engine oil with higher specifications: Typically synthetic lubricants.

The effects of high temperatures on motor oil in turbocharged engines.

In an endeavor to achieve the best possible friction
reduction at normal operating temperature, oils are formulated with specific base oil stocks and additives in order to carry out specific jobs within the engine. These oils are designed to function in an optimal way at the normal operating temperature range of the engine. As a general rule of thumb, an engine oil’s operating temperature should be approximately 10°C to 15°C above the cooling water temperature.

When an oil is heated up, its viscosity will decrease. The formulation of the oil is such that it keeps the following in perspective:

• The oil must be able to be pumped through the system as fast as possible when cold on start-up in order to lubricate all components within a few seconds of the engine starting. 
• The same oil must then be able to supply sufficient lubrication to all components at normal operating temperature. 
•  Different environments will place different demands on the lubricating oil but it must remain ‘thin’ (low viscosity) enough when cold so that it can flow and ‘thick’(high viscosity) enough when hot to keep moving parts separated.

As mentioned previously, an oil’s viscosity will decrease as temperature increases.

A standard 15W40 crude-based multigrade engine oil,the grade commonly used in Southern Africa, has a starting viscosity at 40°C of approximately 110mmSq.mm/s (centistokes) and an operating viscosity at 100°C of approximately 14.5Sq.mm/s.

Another general rule that can be applied to the operating viscosity of engine oil is that, if the operating viscosity of the oil goes below 10Sq.mm/s, then the oil is too thin to lubricate all components within the engine adequately.

With turbochargers reaching temperatures of 800 deg C (And higher) regular engine oils become very thin and because of the high rotational speeds of the turbo, can lead to excessive bearing wear or even failure.

 However synthetic motor oils are far more stable at high operating temperatures that turbochargers operate at, and offer superior protection.

At these elevated temperatures there is also an increased danger that (In particular) mineral motor oil can carburise: The carbon then acts as a grit on the bearing rapidly destroying the surface which results in excessive clearance between the shaft and the bearing leading to turbo failure.

What API rating would be best for turbocharged engine motor oil.

The recently introduced API Service Category (API SN) was developed with turbocharger performance in mind during development of the category. Oils meeting API SN should at least provide a fair level of protection for turbocharged engines.

With a turbocharger repair being very costly it makes sense to use only the best motor oil for turbocharged engines. Habot Synthetic Lubricants produce a full synthetic motor oil that meets or exceeds all OEM specifications.