The most critical factor is the SAE Viscosity Grade (e.g., 5W-30, 10W-40), which determines flow characteristics at specific temperatures. For modern engines, you must prioritize Synthetic or Semi-Synthetic oils over conventional mineral oils to ensure better thermal stability and oxidation resistance. Additionally, verify the API (American Petroleum Institute) Service Category—ensure it meets the latest standards like API SP for gasoline engines or API CK-4 for diesel engines to guarantee compatibility with modern emission control systems.