I am currently involved in a project of installing mini-mess sample ports in the gearboxes of plant I work at. Each unit has to be looked at to determine the best location to put it in. But for most applications I will place the valve in the drain port. I am using the type with the swivel tube extend to place the end of the tube into an active area of the oil splashing around inside the gearbox. By bending the tube slightly away from the bottom of the gearcase we are not sampling the sludge which collects there. On circulating systems you ideally want to place the sample ports in return lines before any filters or the reservoir.