There is disclosed an improved tool, e.g. adapted to comprise at least part of a well completion assembly or well drilling assembly. The invention provides an improved downhole tool (or device) having a friction factor of the order of ten times less than those known from the prior art, e.g. of the order of 0.100 or less. Accordingly the invention provides a downhole tool (10; 10a; 10b; 10c; 10d), at least part of the downhole tool or device being made from Tungsten Disulphide (Tungsten Disulfide). In a disclosed embodiment the at least part comprises at least one surface of the downhole tool, the at least one surface comprising a bearing surface, e.g. a journal bearing surface and/or a thrust bearing surface.