I've heard this case made many times by many people on here, but I have never observed this myself in any Honda I have owned or ridden.
Maybe I'm just not sensitive enough? Most of us change the oil every 1000 miles or so. Oil should not be significantly degraded in that mileage to the point that the rider could detect changes in viscosity. Bikes with no oil filter do start to accumulate suspended combustion contaminants in that mileage which should be removed by draining the oil, but the lubrication properties should not be degraded to the point of causing shifting problems or causing abnormal engine wear due to viscosity breakdown. Anyway, I guess YMMV.
This subject is near & dear to me, and possibly could be a good topic for another thread, as my Rebel 250 has had a long history of oil thinning, only getting around 500-600 miles before shifting get difficult. The color of the oil looked good, but just came out thin. Another trait my bike had, was the amount of crankcase vent discharge was relatively high, so I had to drain the tube often. I remember reading a segment in the owners manual which informed that this discharge will increase if driving hard, and/or driving in high humidity or rain. So I suspected I was just driving it to hard, because humidity is relatively low in NW Minnesota.
I don't believe the oil thinning was due to water moisture build-up in the crankcase, because the old oil was not milky in any way. Also, I've heard when a carburetor float sticks, overflowing fuel can find it's way into the crankcase via the carb&intake system. I eventually changed the carburetor, but the problem persisted.
Finally did a valve clearance check, and found the exhaust valve barely at, or slightly, under the minimum acceptable gap of 002. Adjusting this to somewhere between .003 and .004 seemed to have corrected the excessive crankcase venting. I don't know if this is just a coincidence, and since I'm not an engine expert, won't try to speculate here, but just sharing what I found with the team. I have not put enough miles on the bike, to see if the oil thinning problem has gone away, but really hoping it has.
...I'm sorry this quote got so long.