That is the Vent Hose.
It is part of the PCV(positive crankcase ventilation) system.
Your PCV valve or PCV valve hose may be clogged up or cracked if there is oil residue in the Vent hose.
Normal PCV operation has the PCV valve/hose sucking air out of the valve cover/crankcase, via the vacuum in the intake while engine is running, the vent hose allows that air to be replaced, so flow in the vent hose should be > to the oil filler tube, not > to the air cleaner.
Check the PCV valve and hose first, 99.9% of the time that will be the issue.
All piston engines(even brand new) have "blow-by", when a cylinder fires some of that expanding gas will "blow-by" the piston's rings, this does two things, it increases pressure in the crankcase and it vaporizes some of the oil on the cylinder walls, this is where the engine oil vapor comes from, oil squirting out of bearings and valve train is more of a "mist" than a vapor so will settle out pretty fast.
If you don't change oil as recommended or use a cheap oil you will get more oil vapor than "normal".
Blow-by increases with RPM(cylinders are firing faster), and as RPMs go up Vacuum in intake goes down, so at a certain RPM(and engine load) there will be a break even point, where vacuum is lower and blow-by is higher, above this point the flow in the Vent Hose can reverse and oil vapor will travel into air filter and into the upper front of the intake.
This can be "normal" under high load and high RPMs, but it would be short duration, so shouldn't show any signs in the intake, unless oil is old and there is alot of oil vapor.
As rings wear down(high miles) blow-by will increase and the break even point will be lower.
Ring condition can be tested with Vacuum gauge or dry then wet compression test.
A general vacuum leak will lower vacuum over all so break even point is lowered.
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