Specific gravity and API gravity are both measurements of density. They both can be used to measure the gravity of a fluid, such as hydraulic oil, in the same way that miles or kilometers can both be used to measure distance. And just like kilometers and miles, neither is more accurate than the other; deciding which to use is primarily a matter of preference and convention.
Specific gravity is calculated by dividing the density of a fluid by the density of water. As you may know, if something is denser than water, it sinks, while if it is less dense than water, it floats. So if a fluid's specific density is higher than 1, it will sink, but if it is less than 1, it will float. Knowing what fluids will float is important in many industrial fields.
Specific gravity is especially important for the petroleum industry. When crude oil is taken from the ground, refineries separate the crude into tar, kerosene, gasoline, hydraulic oil and many other compounds. The knowledge of specific gravities and boiling points allows refineries to separate these compounds efficiently. But values of specific gravity vary widely, and the American Petroleum Institute (API) found it convenient to create a new unit, the eponymous API gravity.
To calculate API gravity, you need to know the fluid's specific gravity. Divide 141.5 by the fluid's specific gravity and subtract 131.5. If that seems arbitrary, it is because the equation was designed for convenience's sake. Values of specific gravity vary widely, but by using this formula, API gravity values of petroleum products are closer together than specific gravity values, making engineers' lives a little easier.
When you slam on the brakes, you are using a hydraulic fluid--brake fluid--to bring the car to a halt. Hydraulic fluid can be any fluid, including water, but if it is a petroleum product, it is called hydraulic oil. "Hydraulic oil" is a catch-all term, and it doesn't have a set API gravity. Instead, fluid with the appropriate API gravity is chosen according to the job at hand.
The most important factor in choosing the right hydraulic oil is its viscosity. Absolute viscosity measures how easily a liquid flows. High-viscosity liquids are thicker--honey's viscosity is much higher than water's. But hydraulic oil is graded according to kinematic viscosity, which is absolute viscosity divided by specific gravity. As a real-life example, using high-viscosity motor oil works better in colder temperatures, whereas using low-viscosity motor oil extends engine life.
