I think that DSDP Don brought up a good point... Making sure that that the batteries are being properly charged... Certainly over charging can cause the out gassing issues... In my case our inverter/converter Chargers are Xantrex freedom 458's.. We have two 2000 watt units... The dip switches are and have been correctly set.
Due to our RVing pattern our batteries are subject to frequent discharge and recharge cycles... We travel a lot and often boondock (dry camp) for several days at a time.
There have been some good points raised relative to the effect that oil would have on the plates if the electrolyte level is allowed to drop to a point where the oil would come into contact... There is also the physics of the catalyst impact of temperature... From what I have read the product I have used is basically engineered mineral oil designed to lesson the impact of temperature as well as reducing the negative consequence of oil contact with the plates by reducing the coagulation or sludge factor...
As I mentioned earlier there was a sticker shock re the cost of the Thermoil product... But... The product has worked for me... The question I have to resolve is whether or I would get the same results from plain mineral oil. Based on my experience I am either going to add Thermoil or mineral oil.
One other comment.. As far as I can determine the adding of an additive where Thermoil or mineral oil its a one time deal... I have added distilled water annually to my batteries but only one application of an additive... I would not and have not added any additional additive when I have added distilled water. I don't think the oil evaporates... Adding would increase the concentration and greatly increase the potential for any potential negative consequence an additive would represent.
JohnnyT