vegetable oil is non conductive. it serves as a passive cooling method. The heat generated from the processor and other parts are absorbed by the oil and it takes a lot of heat to heat up the oil. With fans, a pc running at full load (or overclocked) will heat up to around 125 F. Immersed in oil, the same pc at full load will only heat up to oil to around 105 F.
i personally like zalmans. they are nice and quiet. benchmarks have shown they work. I personally dont like the jet engine fan type of fans cuz they dont work as efficient they are more for looks and they are loud.