I bought my 400 last summer, and it had (and still has) a little oil weeping. I had a similar experience as you - I cleaned the thing and then noticed a little oil here and there on the fins. After 3 or 4 weeks of riding, met an old timer who gave me some info I found helpful, but mainly I found it just made me feel better.
I can't vouch for the accuracy of what he said, but it made sense to me, when I thought about it. He said that when those machines were sold new they came with a kind of a service schedule from Honda. In that schedule, sometime during the first month or two (or around a thousand miles or so) the new owner was required to take the machine in to his dealer for service. One of the things the dealer was supposed to do at that time was take off the top of the motor (whatever that's called - the head or something) and re-torque a series of long bolts which hold different parts of the engine together transversely. This re-torque was pretty fundamental, and important. It couldn't be done at the factory because the motor needed to run for a while in order for the procedure to be effective. But once it was done at that particular time, the motor would be good from then on (barring undue maltreatment or abnormal stressing).
The catch, according to this guy I spoke with, was that some owners never took their machines in for the servicing, and some dealers didn't perform the re-torque properly or at all.
So over the years i imagine some of the machines which didn't get that service item performed probably didn't age under as ideal a circumstance as they would have if they would have, had the particular service been performed.
This man had had Honda 4's of different sizes for about 35 years when I met him. He went on to say that a little weeping, as he called it, is normal for the 4 cylinder Honda engines. Not a lot, but a steady little. He went on to explain the difference between an oil leak, and this little "weeping" business. My weeping comes and goes, depending on how much and how hard I ride. But it's definitely weeping, not leaking. I just wipe it off (although I do watch closely the location and intensity - so I can jump on it if it moves up to a bona fide "leak").
I mention this not because I think it's particularly helpful to you, but because it helped me understand the nature of these great old engines a little better. It certainly made me feel a lot better about wiping away evidence of oil as well - wiping it away before I was told that a little weeping was "normal" (or at least that it wasn't a terrible thing at the degree to which it was happening in my engine) made me feel like I was just denying a real problem.
I suppose there are other old timers who might say this is all b.s., and that these engines are not supposed to leak at all, under any circumstances. But it makes sense to me.
Once I get comfortable enough to actually go inside my engine (next winter, most likely) I'll be able to check the torque of the bolts this old chap mentioned. But since I spoke with the guy I've put almost 20,000 kilometers on the machine and it still runs like a dream. I change the oil religiously (maybe even too often) and I estimate, depending on the intensity with which I ride, that it might "weep" a dozen drops or so, between changes. (Hard to calculate the amount because the oil never drips (that would be a leak, I guess) it just shows up on the fins. If I don't wipe it away it collects dirt and grime quickly, and then it looks a lot worse than it is.
Sorry for the over-long post. Just thought I'd tell you that little story, for whatever it's worth.
Best of luck,
Z.
