When sourcing base oil, you must prioritize the API Group classification (Group I, II, III, IV, or V) as it determines the refining process and performance level. Key parameters include Viscosity Index (VI), which indicates stability across temperature changes; Flash Point, ensuring safety during high-temperature operations; and Pour Point, which is critical for cold-weather fluidity. Additionally, check for Sulfur content and Saturates levels, as higher saturation typically leads to better oxidation stability and longer lubricant life.