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Brake oil is used to transfer hydraulically the pressure from the master cylinder of the pedal (normally enhanced by means of a vacuum servo) to the slave cylinders and pistons of the braking system.

The requirements of the brake fluid are: low viscosity, thermally stable and high boiling point. There is virtually no requirement for the fluid to provide a lubrication action. The need for the thermal stability is due to the heating effect of applying the brakes which - in either heavy use such as racing, or in mountainous terrain - can cause a significant thermal transfer to the fluid itself (don't forget they operate on friction). With some brakes glowing "cherry red" or even hotter, the thermal transfer risks become clear.

Transmission and differential oils are both used to provide a lubrication as a primary function to minimaise the wear on the gears.

Engine oil has another requiremment. The main bearing journals on an engine along with the big-ends (and some others too) are "float type" bearings where the oil is pumped in at high pressure and create a hydraulic float preventing the metal components from touching in use. To do this, the oil has to be of sufficient viscosity to permit pumping to the desired areas and stay in the journals long enough to provide the required cushoning. In additon, there is a cooling action provided to the bottom of the piston crowns, normally by means of "directed spray" and additional feeds to the small ends, camshafts (which often also have journals) valve train, etc etc. Many engines now also have hydraulic tappets to optimise the engine performance throughout its lifecycle, which require a lower viscosity.

Indeed selecting engine oil is a very difficult and complex balancing act to ensure the appropriate compromise engine-wide to minimise wear, optimise component life and meet all the necessary functional requirements. As can be inferred from above, when the engine is stationary, the journals empty of oil and you have metal on metal contact which is by far the most damaging to the engine...the cold oil has to be low viscosity in order to fill the journals as fast as possible to prevent failure, yet have a high boiling point as it is used to cool the underside of the piston crowns. Equally, it must be fluid at significantly sub-zero temperatures for use in upper (or lower) latitudes in winter and even at high altitude.

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