The correct answer is C) heated to reduce viscosity. Lube oil filters are more efficient at removing contaminants when the oil being filtered has a lower viscosity. Heating the oil reduces its viscosity, making it easier for the filter to capture and remove particulate matter and other contaminants. Lower viscosity oil flows more readily through the filter media, allowing the filter to capture more of the impurities. The other options are incorrect because high pressure (A) and low pressure (B) do not directly affect the viscosity of the oil, and cooling the oil to increase viscosity (D) would actually make the filtration process less efficient.