Hmm you should not be able to hear a drip on the inside externally, there are lots of places you could have a small leak and never see it. Dark oil after a rebuild is very possible, depending on how dirty your engine was. But if that image is the colour of your oil you have a problem, that yellow brown colour is a sign of be coolant contamination.

But but takes a lot of water to get that colour.

 

Just wanted to add if you did not get that valley pan gasket set right you will get water in the oil. Those water passages at the ends of the block could leak, and the water will run into the valley and then into the sump. You will to pull the top end, but not teh heads. Before that use a product like the one below :

The only way to find out for sure is to get a fluorescent dye kit