The mineral oil trick is to put about 1-2 oz of mineral in each cell - I think I used 1.5 oz. The idea is since it stays on top of the water it stops evaporation and gassing.
I had read about it and most gave it good reviews. I figured I'd give it a shot. Best case it worked as suggested, worst case I'd shorten the life of some cheap batteries.
So far I'd say it was a success and I'd do it again. Maybe I won't feel that way in 3-4 more years, but the early results are encouraging. I'm 2 years into it now, they're still clean, they're working well, and I haven't touched them since they were installed. I'm happy.