I have never been convinced that significant oil seal failure is ever really to blame for oil leakage. The crankshaft is well above sump oil level, so there would surely only ever be a tendency for oil to leak past the seal if the crankcase became pressurised?
In fact, the opposite is the case as long as the ventilation system is clean and operating properly, because crankcase air is constantly being drawn out via the inlet manifold. If there is negative pressure in the crankcase then how can a seal leak?
I recall older cars that didn't have crankshaft seals at all, just a close tollerance and a scroll on the output end of the crankshaft which tended to wind oil back into the crankcase. You centralised the housing using a feeler gauge.
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