Once again - 1992 Ford F150 5.0L 302 V8 175k miles, auto.

A mechanic determined there's a blown head gasket.
Following behind the truck, you could see "steam"
coming out the exhaust and the truck ran roughly.

Upon disassembly of the manifold and heads, we see:
* the inside of valve covers are coated in brown goop,
  which is obviously water mixed with oil
* the rear of the lifter valley has the same muck
* the underside of the (lower) intake manifold, at the rear,
  is coated with the same brown goop.

Cylinder 3 (I could be wrong - it's the 3rd cyl back from
the front on the passenger side) has, what appears to
be either "a build up of something" or scoring from the
piston.  Looking at it, you want to rub your hand on the
surface to rub off whatever it is, but it wont.

Anyway, so here's the strange part - examining the head
gaskets, there is no visible sign of a "break" or "blown"
pathway. Examining the "impressions" on the head and
the block from the gasket (the "ring" impressions), there
is no evidence of water traveling to a piston bore/etc.

Any ideas what this could possibly be?  Could there be
a hairline crack in the block or head or intake manifold?
Or could there be a hairline tear in the gasket that is not
visible to the naked eye?