Viscosity is defined as the internal friction or flow resistance of a fluid. Viscosity in mineral oils is an important property that directly affects the performance and application of the oil. Accurate determination of viscosity helps predict how oil will perform at a given temperature. Therefore, the determination of viscosity in mineral oils is a critical analysis to determine the quality and suitability of the oil.

Viscosity is usually measured by two main methods:

1. **Kinematic Viscosity:** This determines how fast the oil will flow at a given temperature. It is measured at a specific temperature (usually 40°C or 100°C) in a device called a viscometer. Results are often expressed as "cSt" (centistoke).

2. **Dynamic (or Absolute) Viscosity:** This indicates the fluid's resistance to flow and is usually expressed as "cP" (sentipoise). It is measured with a device called a rheometer.

**Standard Methods Used for Viscosity Determination:**

- **ASTM D445:** The most widely used standard for the determination of kinematic viscosity. This test uses a capillary viscometer to determine how fast the viscosity will flow at a given temperature.

- **ASTM D2270:** This is used for viscosity index (VI) determination. The viscosity index measures the viscosity response of the oil to temperature changes.

Such analyzes are important to determine the correct applications of mineral oils, to optimize their formulation and to ensure product quality. The viscosity of the oil must comply with certain specifications and standards for motor oils, hydraulic oils and other industrial oils. Otherwise, it may have adverse effects on the performance and life of the equipment.