mineral oil in rv batteries

I bought one 12 volt deepcharge batterie some 4 or 5 years ago that they where selling along with solar pane. It came from factory with oil in it. Do not know how much or what type of oil, but it worked. That battery is still very good and I do not remember having to add water, it says to check it and refill with demineralized water. The oil floats on top of water and it prevent loosing electrolyte by evaporation. It also says that you can use it inside with no danger with evaporation of dangerous fumes.

I have read that some long time ago, they where using oil in batteries. If you have the right oil in the right amount, I would say it is a very good option.