The root cause of the oil leak could be a few things.
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Boost leaks, causing the compressor to over work and eventually creating a vaccuum in the compressor housing sucking oil past the seal.
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A restricted oil drain. How does the turbine side look as well?
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Does this turbo have any shaft play indicating bearing wear?
Odds are being an old (looks like off an audi or porsche) KKK that it probably had boost leaks somewhere in the system or also if off an old air cooled porsche possible that the oil was old, hot and coking. Causing the seal to just plain wear.
Either way if you need it rebuilt stop in at D+W diesel, there is one in rochester and they can rebuild it for you as they are KKK/switzer dealers.