The Rohloff Hub and Oil - Doing the Math
An issue that often results in a call or service visit from Rohloff hub owners is oil leakage from the hub. This is also probably the issue that is least significant in terms of hub function and wear. Anyway, here is my take on oil leakage from your Rohloff hub:
How does oil leak from the hub? The innards of the hub are not tightly sealed from the outside environment. In other words, oil can seep out of the seals (and moisture can seep in). This can result in oil appearing on the outside of the hub, in the hollow quick release axle, and in/on various associated parts such as the external gear mechanism, cables, the floor, etc.
Why does oil leak from the hub? I think the most common root cause is overfilling of the hub (see arithmetic below). However, there are external factors that facilitate oil leakage, to wit, ambient temperature/pressure changes and/or the physical storage position of the hub (on its side). Also, if the oil is thinner than normal for some reason, such as incomplete drainage at the time of oil change, it might be more likely to leak.
Arithmetic: First of all, I think the Australian Rohloff distributor has a decent summary of oil leakage and math on their website.
The essentials are: 1) 7 ml of oil are needed to provide the actual lubrication; this amount of oil adheres to the internal gear surfaces as a result of surface tension, and 2) 15 ml of oil is the optimum amount of free oil in the hub to provide noise suppression. When oil is first added to new hubs, many mechanics simply add the entire amount of the oil in the bottle supplied by Rohloff. This bottle contains 25 ml of oil. So, after the initial oil is added, 7 ml of oil will be adherent to internal parts and 18 ml of oil will be free in the hub, minimally over the optimal amount, but still good.
Assuming no leakage or loss for other reasons, at the time of oil change, after adding the cleaning oil, and after fully draining the hub, there will still be at least 7 ml of oil in the hub, this residual adherent to the internal gear surfaces. So, if one then adds the full contents of the 25 ml lubricating oil bottle from Rohloff, the hub at this point will contain 32 ml of oil, 7 ml adherent to gear surfaces and 25 ml free; in other words, the hub will be over filled by about 10 ml. That excess 10 ml of oil might not cause any issues, but given the opportunity, it will try to leak out. Opportunity, as mentioned above, could be changes in ambient temperature/pressure, and/or wheel position (lying on its side).
So, when filling a new hub for the first time, it's ok to put 25 ml of oil into the hub, but when adding oil during an oil change, after thoroughly and completely draining all cleaning oil and old lubricating oil, no more than 15 ml of fresh lubricating oil should be added.
As mentioned, common reasons for oil to leak out of the hub include changes in ambient temperature/pressure or storing the wheel on its side. Temperature and pressure changes can be associated with riding conditions (for example, riding in hot weather), or (and this is common) flying with the hub. Also, when flying with the hub it is essentially impossible to keep the wheel upright, so oil might be able to pool over seals and, with pressure changes during the flight, be forced out through the seals by decreased pressure in the plane, in other words a double whammy. This is why, when I fly with our Rohloff bikes, I wrap the hubs in rags and place large, absorbent pipe cleaners in the hollow axle, to soak up any seeping oil. And remember, failure to fully drain out the cleaning oil and old oil mixture during an oil change might result in oil that is thinner than normal and therefore more likely to leak out through seals.
There are other possible causes for oil leakage, such as damaged seals or a loose axle plate, but these are less common. When an oil leak is discovered, inspect your hub to be sure it has no obvious damage, but don't fret too much. It probably leaked because of reasons detailed above, and even if all the free oil leaks out, 7 ml will still be retained in the hub via surface tension, enough to provide the needed lubrication of the internal parts.
To summarize the math:
Amount of oil the hub holds onto via surface tension: 7 ml.
Amount of oil necessary for lubrication: 7 ml.
Optimum amount of free oil in the hub: 15 ml.
Purpose of having the free oil in the hub: Noise suppression.
Amount of oil to add to a new hub at the first filling: 25 ml.
Amount of oil to add at the time of oil change: 15 ml. **
**Note: at the time of oil change the hub should be thoroughly drained of all old lubrication and cleaning oil prior to adding new lubricating oil.