When these parts are (were) new, they had a Parkerize-like finish. This powder-like finish grinds off like a lapping compound in use, polishing the mating surfaces to make them friendly with each other. It also polished the gears in the oil pump way back when, to make them into mated impellers, as it passed through those regions. Eventually, the grit all ended up in the first oil filter.
The non-meshing parts don't grind the finish off. Instead, it retains a thin coat of oil (and moisture if left sitting too much) that allows for an extra oil feed as the gears speed up from a standing (or low-speed) start. This is part of how the gear teeth and dogs are lubed during the up & down RPM changes of city riding, and why the placement of the 4th and 5th gears are where they are, receiving the lion's share of the splashing oil (because they don't get the 'freebies' of the rough finish for very long).
Over the years, which are quite a few more than Honda expected, I'm pretty sure...the finish also retained the water vapors of idle use. This promotes some rusting in bad cases. This isn't all bad: this fine grit will restart some of the polishing business again that can help when new(er) parts are added to a used engine. It only gets one trip though the oil pump, so it doesn't hurt things very much in the end. If it is real thick and can be scraped off the unused surfaces with your fingernails, then consider at least a wire toothbrush to remove most of it, lest your oil pump get more polishing than it needs now.
If you are installing other used parts, don't clean this grit off. It will help the new pieces meld in over time, albeit a slow break-in. Totally new engines do this process in about 500 miles, while well-seated engines with new parts can take 2000 miles before they smooth out in the tranny area.
The shift drum, like Old Scrambler said, can be improved (sort of) in the K2. The K0-K2 bikes (and early K3 old factory) had the 2nd revision of this drum (sandcasts had a real early one besides this one) that make for real smooth 1-2 shifts, but not a real easily-found Neutral. It also, after some wear, can develop a spare Neutral between 2-3, which can be confusing when the Neutral light also stops working...like mine...the 1-2 "notch" in the drums that came out after the K3 have a wider shelf to expand the Neutral area. They feel notchier, though, and are not well suited to fast, slick shifts like racers desire. The 2-3 Neutral rarely appears on these drums because the ramps are steeper between the gears and the corresponding pins on the forks are longer and narrower by a hair. All the parts interchange, though, except for the sandcast drums.
The lapping compound in the gear teeth is a special situation, suitable for those who have 1.) lots of patience to take the time for it and 2) the desire to extract the lost HP from the gearbox. This involves putting lapping compound directly on the gear teeth to accelerate their root wear so when the shafts twist under power, the teeth roll non-parallel to one another with a smaller contact patch. This box has teeth about twice as wide as needed, so partial tooth engagement has NEVER been a problem.
Partial dog engagement is another story, however. The L shift fork in particular can become worn, either from too light or poor choice of oils, or from lazy partial shifting into 2nd, which makes the gears try to push back to Neutral, leaning on the fork in the process. The box willingly will do this, especially on the K0/K1/K2 with the "Type 2" (as I call it) shift drum. Over time this rounds off the leading edges of the dogs on the C2-C5 engagement pair, leading later to jumps-out-of-2nd-gear problems. A slight back-cut on the dogs and slots will prevent the push-out of a partial shift as it tends to pull the gears together when they start to engage. We used to call this "pull cutting" the dogs, everyone now calls it "back-cutting", same thing. If you're not real savvy with the process, it is best left to folks like APE, who can do the C2-C5 pair for about $250, absolute first-class work.
The changes that became the "Type 3" and most common shift drum were accentuated in the F1 bikes, so the later shift drums give real positive shifts. They also require the clutch work right, with good release, as any drag will make it hard to upshift. This has caused other symptoms we sometimes see, to wit: the pin plate and pins on the shift drum get worked loose from the constantly too-stiff shifting, and pretty soon it is almost impossible to hit 2nd or 3rd gears. If this happens, it is best to replace the drum, as the pins then make their holes oval and even if retightened, will work themselves loose after a while.
