Here's my story.
I have a stroked 392 Ford Racing Motor with a Holley Street Avenger carb that came with the build. Breather on one valve cover and PCV on the other (stock type of PCV that you find in the parts store). Well I decided to swap out that ugly looking PCV with an aftermarket PCV that had a chromed housing and looked identical to the breather...and looked much more attractive on the valve cover. Within a couple hundred miles of driving, I began to notice a tear drop of
oil at the bottom of the bell housing/engine backing plate -
oil coming from the rear main seal. I swapped that 'ugly' PCV back in and tossed the chromed aftermarket PCV. Haven't had a drop of
oil since.