Most of the time I work on rack systems, but today my supervisor asked me to replace the filter drier of an old light truck thermoking unit. This is the first time I get involved in such transport systems. He hinted me that some oil must be added to the new drier in order to account for the one being lost by discarding the old unit. But how much should that be? Also, why can´t the oil be added to the compressor instead?