What Eljaro said...
I went through this with the center oiler in the batmobile, it's pretty unlikely that any sort of condensation / humitidy would cause the change in
oil texture. You sould change the
oil and pull the valve covers, you will likely find there is a good deal of moisture collected in the valve cover roof.
It could be lots of things. In my case fortunately it was the water passages at the heads / intake manifold connection leaking. It turned into a little bigger project than expected when I found a huge mismatch between the standard Edelbrock F-427 and the extensively ported 427 Sideoiler heads. If it isn't obvious things get a little more involved, chasing cracks, replacing head gaskets, etc.
Good Luck,