Gifford repeated exactly what I recall hearing from my Father and Grandfather.  Oil and grease attract dust, which eats the bearings quicker.

Our big Massey Harris combine had a Scour-clean, which had wood bearings, the only experience I ever had.  After many years, they did go out, while I was taking wood shop in High School.  I was going to make some, but shop teacher advised we could not find the proper wood.  He suggested apple, but I bet Gifford is right on the Osage Orange, or Hedge (as we call it). 

At that time, we were able to buy those bearings from parts.