Shimano now offers an oiling kit for their hubs. Per their write-up on oiling the hubs you remove the innards from he shell and dip them in the oil for some time, then lift out and let it drain off the excess and reinstall into the shell. Nothing about degreasing first in their recommendations. Here is a link to their oiling instructions.
http://www.shimano.com/publish/conte...ance%20Oil.pdf
When I asked about it to SRAM Tech Support I was told that their hubs can have oil added too if shifting is sticky. Their email indicated that it is suggested for improved cold weather shifting. I would not be surprized that it could improve all shifting and operation regardless of temperature.
As generally speaking IGH hubs are pretty similar in design and internal material used in manufacturing I would expect that about any greased hub could be converted to oil lubrication.
Grease lubrication for IGH units was most likely introduced to take care of the IGH owners who ignore normal maintenance as factory greasing will last longer than oiling but is not really as good a lubricant IMO. An idiot proofing measure.
The Rohloff and I believe the NuVinci CVT both use oil bath lubrication. They have seals designed for a true oil bath lube system though so it all stays inside.
If/when I get another Shimano IGH I will add an oiler to it before building it into a wheel.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
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http://groups.yahoo.com/group/Geared_hub_bikes/