Omga 75 Cu. Ft. tank gauge

Sounds like the gauge has an internal leak (it is a curved brass tube usually soldered - the needle is attached to the end of it via gears).

The feed to the gage has a tiny metering hole and the gauge itself is oil filled in case the gauge tube ruptures - both are to prevent you from getting hit by a flying lens out of an exploding gauge if/when a leak were ever to happen.

Instead you'll get glycerin oil oozing out. In any case - you likely need a completely new gauge as they are not normally repairable.