The difference in specific heat is only a factor of 2. The big difference is in the density, which is about a factor of 1000 larger for liquids over air at atmo

The difference in specific heat is only a factor of 2. The big difference is in the density, which is about a factor of 1000 larger for liquids over air at atmospheric pressure. (However, mineral oil has much higher viscosity than water, which I'm not sure is what you want as it suppresses convection. I guess it's a question of whether you take that over the non-conductivity.)