I use mineral oil. THG used vegetable oil. Transformer oil is generally mineral oil based. I considered mineral oil based hydraulic fluid, but decided against it due to detergents in the fluid. There are some more exotic/expensive options out there, one from 3M. Basically, as phreejak mentioned, it has to be non-conductive. Mineral oil was my least costly option. Always do a test with a multimeter before you chuck your MB in anything, however.
Here's a link to some exotic 3M stuff, for entertainment value -http://products3.3m.com/catalog/us/en001/oil_gas/specialty_materials/node_J6G6P86FLXbe/root_GST1T4S9TCgv/vroot_G1F6DNZDBVge/gvel_TK2003TWNTgl/theme_us_oilgas_3_0/command_AbcPageHandler/output_html