🔍 Key Concepts

• Viscosity index (VI) measures how much an oil’s viscosity changes with temperature • A low VI oil’s thickness changes a lot between hot and cold; a high VI oil’s thickness changes less • Hydraulic systems need reasonably stable viscosity across the operating temperature range

💭 Think About

• Which end of the viscosity index scale (low numbers or high numbers) represents oils that change viscosity more with temperature? • If an oil becomes very thin when hot and very thick when cold, does that mean its viscosity is changing a lot or a little as temperature changes? • Among the choices given, which value best represents an oil whose viscosity is highly sensitive to temperature changes?

✅ Before You Answer

• Recall that higher VI = more stable viscosity with temperature; lower VI = larger change in viscosity • Compare each choice (20, 40, 60, 80) to decide which is the lowest viscosity index • Confirm that the option you pick matches oil that is very temperature sensitive (thin when hot, thick when cold)