If you change the fluids in your bike yourself, good for you! Yes, it's often a sloppy, dirty job, but you get the satisfaction of not only having done it yourself, but in having saved money in the process. Harleys have three (3) drain plugs - one each for the Engine, the Primary, and the Transmission. The fluids in each of these should be changed on a regular basis, according to the mileages listed in the Owner's Manual. Harley standardized on a single size drain plug for all three areas starting in 2007. If you look at the drain plugs, you'll notice that Harley included a small magnet in each one. Theoretically, that magnet should attract and hold any metal particals that pass by them, thus helping to prolong the life of the your bike. In practice though, the magnets Harley includes are so weak they can't even support their own weight! That's not exactly encouraging.
There are replacement drain plugs available with much stronger magnets. One Company making them is Dimple Motor Oil Magnets. They make a large assortment of these for different vehicles, Harleys included. I have a pair of these drain plugs and can say with certainty that they are MUCH stronger than the magnets in the stock drain plugs - And I will be installing them at the next fluid change.
Dimple #SS1/2"x20x1/2" for Harley 2007 up.
While we're on the subject of drain plugs - what about that O-Ring seal? Harley uses a material called Nitrile (Buna-N). It's cheap, and it works, but it's really not as well suited for this application as would be expected. Buna-N is susceptible to high temperatures and some fluids, like brake fluid. A better alternative would be to use a more temperature and fluid resistant material, like Fluorocarbon (Viton). While Viton is a bit more expensive, it has better heat and fluid resistance. You can find Viton o-rings for Harley's drain plugs on Amazon and a host of other places. I've been charged $1 each for Nitrile drain plug o-rings at bike stores, but you can get 100 Viton Harley drain plug o-rings online for $8. If Viton is good enough for the Space Shuttle, it's good enough for me - and that IS Rocket Science!