As I am probably a disappointing response in that I am writing to as for more information. In that I am not familiar with the T1600 and the type engine you are discussing. I do have a Kubota T Series, and is the older 2007 T1770 Model with the Kohler Command CV 17 Overhead Valve Twin air cooled 17 HP engine.
On the engine the air-oil separator canister is mounted on the right valve cover with only a rubber grommet holding it on. It has a vacuum hose going from the top of this canister into the carburetor venturi, to remove oil vapor from the engine.
I have know owners of these engines to find a leak around the grommet, leaving oily residue around that cylinder. Later, I believe Kohler added a bracket to the canister to assist in it's security.
Is this anywhere like the problem your having with "oil leaking from the breather tube"?
Where is this breather tube attached on each end?
Is it a "seep" or more of a "constant drip" of oil and at all engine conditions, idle, under load, etc.
Kohler cautions how critical the oil level is, and to not over-fill. That sometimes can cause oil problems.
jamesemu