No, you should not use standard WD-40 on a toilet fill valve. Standard WD-40 is a petroleum-based product that will degrade, swell, and destroy the rubber seals, washers, and diaphragms inside modern fill valves (such as FluidMaster units), leading to leaks and premature valve failure. If any lubrication is required on plastic moving parts, plumbers recommend using a silicone-based grease or spray that is safe for rubber and plastics. Verification: Check fill valve components for smooth movement without applying harmful petroleum solvents.
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