I've been messing around with geothermals on a server with a friend and, after running out of lava we decided to try piping oil to our geothermal array. It burnt it and gave 20EU/t per bucket for a total of 20,000 EU (same as lava).
Then eventually we tried creosote oil from railcraft... then petrol then finally ,water.
The geothermals accepted all liquid (including water) types via Buildcraft pipes and all gave the same amount of EU per bucket. I'm guessing this isn't supposed to happen. 