I would hold off on adding the mineral oil. While I did add oil to one of my sets with no negative effects, there are a few valid reasons to do with caution.
1. While Edison did recommend adding oil early on, this recommendation seemed to dissapear in the later years of manufacturing.
I am not sure why this practice stopped.
2. "Mineral Oil" is a very broad term. I think the chemical composition varies widely.
3. There is this process called saphonization used in soapmaking where alkaline solutions are used to break down fat. Potassium hydroxide
could break down some oils.
4. If the cells are charged each day, the resulting outgassing of hydrogen and oxygen is likely to prevent carbon dioxide from entering the cell anyway.
5. Even if the perfect oil is found, what happens if the electrolye gets low to the point the active plate material is immersed in oil, does it render that area
useless ? I don't know.
For now I am holding off on using oil. The next thing I will try is to insert a low sulphur one hole rubber stopper in each cell then run a piece of tubing
from the stopper to a container filled with water. This will act like a trap and allow battery venting, absorb mist "carry over", but not allow CO2 to go
back into the cell. The tubing will be arched upward as to not permit the water to be drawn back into the cell when it cools. The oil I used was the type
used in ammonia refrigeration systems. As ammonia is a strong base, I thought breakdown was unlikely. While it did float on the oil, the clear electrolyte
became yellowish indicating some solubility of the oil in the electrolyte. Also, if you overcharge a cell with oil added. it makes a huge foamy mess that is
difficult to clean up.