The correct answer is C) better visual metering adjustment. The vacuum feed sight glass indicator used on cylinder lubricators provides a better visual metering adjustment compared to the discharge side liquid filled type sight glass. This is because the vacuum feed system creates a clear, direct view of the lubricant flow, making it easier for the operator to observe and adjust the lubricant delivery as needed. The liquid filled sight glass, on the other hand, may have some distortion or interference that can make visual monitoring and adjustment more challenging. The other answer choices are incorrect because they do not directly address the key advantage of the vacuum feed sight glass in terms of visual metering adjustment. Option A is about using a lower grade of cylinder oil, option B is about adjustments not being required, and option D is about the number of moving parts, none of which are the primary advantage of the vacuum feed sight glass.