OP wants to spend several hundred dollars on immersion cooling of a $30-$80 induction heater?
Upgrade the heatsinks, MOSFETs, add a fan, buy the next most powerful heater; anything besides immersion cooling, and definitely not with mineral oil. Fercryinoutloud that stuff only works as a decent heat transfer medium when there is a massive temperature differential, like going from windings at 180C in a distribution transformer to a 30C ambient (and using the thermosiphon effect to pump the fluid).
Oh, and mineral oil does, indeed, wick up stranded wire cables like a mo-fo.