I pulled the air filter from my 1998 F-250 (LD) 4.6 liter to inspect it. I found it a red substance along rubbery sealing gasket. It is originating on the "dirty" side of the filter and collected on the edge of the filter that was oriented down. The substance is a little bit sticky and I was able to wipe some of it onto a paper towel. Smells solvent-y. I don't see any on the media.

Any idea what it is and how it is getting there?

 

Probably the glue getting absorbed into the softer foam base, or having a bit of chemical reaction with it. I'd change it out for a new one. If that glue fails it offers a route of unfiltered air into the engine.e

 

Is there a path that vents the transmission into the air intake?

If there is, where is it routed?

 

I'm going to replace it.

 

Not that I'm aware of, but if you think it's ATF, note that the power steering also uses ATF.

Do you have a transmission or power steering leak? If so, I suppose it's possible that the spray is getting sucked into the intake.