The correct answer is C) better visual metering adjustment. The vacuum feed sight glass indicator used on cylinder lubricators provides better visual metering adjustment compared to the discharge side liquid filled type sight glass. The vacuum feed system allows for more precise control and monitoring of the lubricant flow, enabling the operator to make finer adjustments to the lubricant delivery. This is advantageous as it ensures the proper amount of lubrication is provided to the cylinder, which is crucial for the safe and efficient operation of the engine. The other answer choices are incorrect because: A) the number of moving parts is not a key advantage of the vacuum feed system, B) adjustments are still required, and D) the type of lubricant used is not directly related to the sight glass indicator.