Per the PDF "Differences Between Hubs" available on the below linked to web page the SG-8R31, 8R36, 8C31 and S501 have improved seals, among other changes. It also states that the new seals are retrofittable to earlier 8 speed hubs.
http://bike.shimano.com/publish/cont...tech_tips.html
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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
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/