What I see in chemistry labs is a lot of bad practice in terms of changes in direction and right angle entries each one of which can lead to air turbulence in the duct and condensation or deposition of particulate.  And I see all the ducts leading from the off-the-shelf hoods are the same diameter indicating they assume all of the ducts are transporting the same contaminants with the same particle size and characteristics.  I smell the work of HVAC engineers here rather than industrial ventilation engineers.  And I must say, I have a problem with using ASHRAE standards for fume hoods and ventilation systems in general.

2.  The second problem is the centerline velocity of the ducts in the lab. That velocity has to be fast enough to keep what ever you are transporting moving rapidly enough not to deposit.  Examples of those speeds are in table 5-1 in the ACGIH Manual of Recommended Practice for Industrial Ventilation.  

Gases, vapors and smoke   1000 to 2000 feet/minute

So to tell you the truth, I would be surprised if you were NOT getting deposition of oil mist in ducts in the labs I've seen.   And people who don't think they have this problem are just getting deposition in locations that don't drip out where you see it.