Question about severity of diesel blow-by in air filter compartment
 

I used to know the answer to this but have forgotten:
1. How much blow-by in the air filter box is acceptable?
2. Is blow-by completely normal in diesels-like an inevitable consequence of owning a diesel?
3. If a 123 or 124 (with 120K on it) diesel has a small puddle of oil in the air filter compartment is that bad news?
4. How long before it gets worse? I once saw a used diesel where much of the air filter was saturated in oil-it still drove fine (although the owner had to change the air filter very frequently).