Torque settings for oil filler and drain plugs? Really?
Firstly, torque settings are determined by a number of factors: the plug material, casing material, fine thread, course thread, washer material (nylon, fibre, rubber, soft aluminium, etc) as well as the mating design.
The rule of thumb is to nip it up and most definitely not over tighten. You should be able to nip it up with just one hand on the spanner using a light to firm pressure. Any more and the washer will distort with resulting oil leaks and there's the risk of thread damage, particularly to die-cast aluminium castings.
Motorcycle racers are required to 'wire' sump plugs to prevent vibration from undoing plugs and dumping oil on the track.
Having worked on the airplanes I've flown for over 25 years, I think I have a pretty good sense of how to tighten fasteners. Yet, the same experience leads me to use published torque values if they're available as they can compensate for factors that are not readily apparent.