When I used to do my Naptha/STB, and I had a lot of success with it mind you - lye was added to a good amount of water and the powdered Naptha was stirred in, which broke it down and made the alkaloids readily available for the Naptha to pull. However the mixture was very watery - in fact it was essentially black soupy water. It was very viscous, and it made decanting difficult - it wasn't a thick sludge like yours, and if I got any of the bottom dark layer in the evaporation dish, it would cause the end result to be oily and nasty. So I had to be very careful.
How exactly do you get away with your decanting being so messy - doesnt any of the oil and impurities get into your evaporation dish when you pour the Naptha out of the mix?
EDIT: I'm guessing because you only use enough lime/water to make it like a sludge, instead of a watery pool, it is very easy to just pour off the naptha since the MHRB "molasses" doesnt move much?