ANS:D - High viscosity index means a large change in viscosity with change in temperature.

The wrong statement among the options provided is: Naphthenic oil has very low viscosity index. Here's why:

• Multigrade lubricating oils have high viscosity index: This statement is generally true. Multigrade oils are formulated to have a high viscosity index, which means their viscosity changes less with temperature compared to single-grade oils.
• Paraffinic oil has very high viscosity index: Paraffinic oils typically have a higher viscosity index than naphthenic oils. This is because paraffinic oils contain straight-chain or branched-chain hydrocarbons that have a more uniform molecular structure, resulting in less change in viscosity with temperature.
• Naphthenic oil has very low viscosity index: This statement is incorrect. Naphthenic oils actually tend to have a higher viscosity index than paraffinic oils. Naphthenic oils contain cyclic hydrocarbons (naphthenes) that can vary in structure, but they generally have better viscosity-temperature behavior compared to paraffinic oils.
• High viscosity index means a large change in viscosity with change in temperature: This statement is incorrect. High viscosity index actually means a smaller change in viscosity with temperature. Oils with high viscosity index maintain their viscosity more consistently across a range of temperatures, whereas oils with low viscosity index experience greater changes in viscosity with temperature.