Why are petroleum-based hydraulic fluids used most frequently?

Petroleum-based hydraulic fluids are widely used due to a combination of factors that make them particularly advantageous for hydraulic systems. One of the primary reasons is their low cost, which makes them an economical choice for many applications. They are also readily available, ensuring that industries can easily source these fluids without significant delays or supply chain issues.

Additionally, petroleum-based fluids exhibit excellent lubricating qualities, which are crucial for maintaining the efficiency and longevity of hydraulic components. The combination of low cost, availability, and effective lubrication makes petroleum-based hydraulic fluids a popular choice in various industrial and mechanical applications, thereby validating the choice of "all the above" as the correct answer. This comprehensive understanding highlights why these fluids are prevalent in hydraulic systems.