Could you please clarify the problem you have encountered? It appears you are questioning the reasoning for an oil filtration system on an SF6 cooled transformer. In this case an oil filter for cooling oil will not be required.
i think you are talking about bushings interfacing SF6 switcggera to the transformer, the bushing is most probably oil to sf6 type or is it of some other type. for oil to air type the online oil filtration system can also be used, as it only keeps the oil in good condition and minimises oil deterioration rate.
please give some more details then only the actual need of the same can be identiifed.
