Just been working on the bike (road bike - lightweight wheels). Was adjusting gears so the bike was upside down (I don't have a stand).

I noticed that when I back pedal the back wheel will begin to spin slowly, which suggests too much friction inside the hub. I took the hub off, cleaned it out, cleaned up the pawls, lubed it up with light oil as you should, and put it back on correctly, but it still happens.

Tried my other bike which is only about 7 months old and in pretty fresh condition and it happens on that bike too.

One bike has mavic hubs / wheels, the other is corima.

I'm wondering if this is just something that is accepted??

All you can really tell is that there's more internal friction in your freewheel than there is in the wheel bearings. This allows the small amount of force transferred by the freewheel to turn the wheel when there's no load on it. It's probably nothing unusual, in fact it could point to your wheel bearings being very low friction, which is always a good thing. As long as it works correctly when the bike is the right way up on its wheels I wouldn't worry.

I didn't think of it that way. Thanks for the advice.

I really wish I was a better bike mechanic and then I'd know all this stuff

It does not work that way. You don't become a mechanic by reading a bunch of books or forum posts, or according to how many years or how many bikes.

What you just experienced is how you become a mechanic, except that you short-circuited the process a bit. You observed what the wheel did when you pedaled backward and wondered why. That's better than just assuming it's OK. But a mechanic's next step is to look at how things interact in order to figure out the why. You thought there was too much friction in the hub, but you need to look at each link in the chain (pun intended) to find out why something happens in a mechanical system. Bear with me while I lay out the process. I'm not trying to be pedantic or patronizing - it's just that a lot of people think a mechanic is "good with their hands" or "has a lot of experience" when those are actually a small part of the story.

You started by turning the crank, which turned the chain, which turned the freewheel, and then the wheel started slowly turning.

Hmm, the freewheel must be turning the hub, but how? It's "freewheeling" and the wheel is turning slower than the freewheel, so of course it's not a direct connection.

Maybe the freewheel is rubbing slightly on the spokes or a spoke guard (does happen).

No? Then somehow the outer part of the freewheel that's driven by the chain must be causing the part attached to the hub to move. The parts obviously slide past each other, but what if the two parts of the freewheel "rub," instead of the freewheel rubbing the hub? That would cause the same effect.

As the Monster Pete pointed out, once that friction is enough to overcome the friction/inertia of the wheel the wheel will turn. If the two parts of the freewheel somehow had zero friction OR if the hub friction was high then the wheel would not turn.

Most likely nobody told Monster Pete what he told you - he figured it out similar to the above, and it's a never-ending process. You will never "know all this stuff," and only partly because things on bikes constantly change, just as with all technology. It's mainly because being a mechanic is not about memorizing a bunch of rules or information, though some things, like the typical space between rear dropouts, the width of a bottom bracket shell, ball bearing sizes, etc. naturally become part of your knowledge. What is most critical is to observe, logically analyze, test and then repeat the process - it's no different than troubleshooting a car, computer or any other problem involving a device with interacting components.

Likewise most of the knowledge you do need has nothing to do with bikes specifically. You don't need formal training in physics, but you need at least a gut level understanding of things like force friction, leverage, and inertia. The same goes for geometry - you don't have to know how to figure out fork trail from scratch, but you need to "know" how moving something to a different angle or changing it's length can affect the parts involved. Finally, there is no way that you will become a good mechanic (or in my opinion even a passable one) if you don't understand how to use and apply mathematics.

You would be surprised at how often those of us who have been at this a long time have to figure out the cause or solution to something we have not seen before. That's the most enjoyable part of being a mechanic.

At a first glance a bicycle is a simple machine but the devil is in the details. I used to be pretty smart and had good understanding of how everything worked, but there are constant detail changes being made in bicycle components. Not only that but the rate of change is faster than I can absorb. The longer I mess with bicycles, the more new things I'm finding that I don't know about so the dumber I'm becoming.

Yep, it's mostly observation and logic. If you're strong enough on those, all you're missing that an experienced guy has is a few tricks of the trade.