The first thing I would check is the oil level. If it is overfilled, the excess will be blown out the breather and into the air filter. Also if the machine is moved around and tipped so that the air filter is downhill the same thing will happen. Clean out the air box, replace the air filter with a clean one and make sure your oil level is correct.
And fix the oil cooler line, or you may run the chance of seizing the entire engine if the oil runs too low
The Engine seems to run fine . It doesn't start on the first turnover but it runs well. There is no smoke that comes out of the exhuast. I have some experience with engines and have checked most of the symptoms that make some kind of sense as to what might be wrong without taking apart everything.
The oil level is actually on the lower side [it doesn't need oil].
I haven't been around when the engine has run for any length of time that is over an hour or so.