The black oily substance is probably the buildup of anaerobic bacteria waste products. I assume that in the time you have owned the house that you have had the septic tanks pumped. You probably should consider having them pumped at least on an annual basis bacause or the age and type of system you have. The drywell can best be treated by having it pumped. Then take a pole or a long pipe that you can force high pressure water through and try to agitate the bottom. This breaks up the anaerobic layer and allows us to treat the system from both sides of this layer. Then add two bottles of Septic Scrub drainfield flow restorer to the drywell. The next morning add two more bottles. This should get the flow started again. Watch the system for the next few weeks. If the water level starts to rise in the drywell, you should consider adding another two bottles. No matter what happens, you should add Septic Scrub about every six months to help keep the system flowing. The goal is to have minimal water in the drywell over a 24 hour period. All of the water you introduce should be absorbed, mainly through the bottom of the drywell, each day. Your description of your system leads to several questions. Where does the pipe from your septic tanks go? How is the drywell connected to the septic tanks? These old homemade sytems for summer cabins in most cases were not designed to handle the situation where they are used every day. No matter what else you do, you should practice water conservation and use the sytem as little as possible until sewers are installed.