this started way before I was thought about. A layer of mineral oil to prevent evaporation. It occurs anyway a degree relevent to temperature.
Most new deep cycle batteries still vent a little. I've installed the remote watering system. They haven't had to add a teaspoon of water, where it is stored since 12. It is on charge controller and plugged in.
Never tied the oil but a lot of Folks dis that I knew. Still had to add some water.
Is it doing anything for the deposits around post and connections?
The maintenance free batteries, or the better of them, don't seem to be any problem.