I was just trying to help Henro, because the title and his posts were kind of confusing a bit.
I appreciate that!
I did mess up in the post title, and a bit in other posts as well, as nothing related to either the EGR or PCV circuits makes any sense regarding making the oil black...
Now
maybe if an exhaust valve guide was leaking exhaust gases into the valve cover, this
might theoretically help to darken the oil, as these gases would find there way past oil as they travel along the way into the PCV path...
In reality, is it fair to assume that black oil is the result of blow by exclusively?
If one engine has slower darkening of the oil than another, after the same period of use, is it likely that the engine with cleaner oil has piston rings that seal better than those in the other engine?
Other than blow by past the piston rings, is there any other reason that might cause engine oil to darken?