Which number indicates only the viscosity of the oil used in fire apparatus?

The number that indicates only the viscosity of the oil used in fire apparatus is the SAE, which stands for the Society of Automotive Engineers. The SAE viscosity grading system classifies oils based on their viscosity at a specific temperature, providing a clear indication of how well the oil will perform under various operating conditions. The grading system includes single-grade oils, which are designated by a number (e.g., SAE 30), indicating the viscosity at a specific temperature, and multi-grade oils, designated by two numbers separated by a 'W' (e.g., SAE 10W-30), reflecting the oil's viscosity at both low and high temperatures.

Understanding this classification is crucial for pump operators, as the viscosity of the oil impacts the performance and efficiency of the equipment, particularly in fire apparatus that require reliable operation under varying temperatures and conditions.