Does this mean that Cetol might make wood more susceptible to mildew than varnish would? My compression post would like to know.
No. Cetol is permeable not porous. A permeable membrane requires a pressure differential for vapor to pass through. That's how Gore-Tex works.
Non-permeable finishes, like varnish and latex paint do not allow any vapor to pass through. All wood contains air and moisture when it heats up in the sun, the air and moisture expands and tries to escape from the the wood. When it hits the finish it puts pressure on the finish which causes to begin to pull away, the finish will bubble. At night the wood cools and air and moisture are drawn back in. The repeated pressure changes eventually weakens the bond between the wood and finish and the finish cracks and fails.
Because Cetol is permeable, the vapor (not water) passes in and out with pressure changes. As a result it does not stress the bond between the finish and the wood. Because of the properties of water, water vapor is able to pass through while liquid water can not.