How you are explaining it, it sounds like you have a head gasket leak. A pic of the sludge would tell a lot. The color and how thick it actually looks. I second pulling off the valve covers and seeing whats going on in there. If you have a coolant leak then it will show in there all white and sludge like as well. Then with those off you can clean all the excess sludge from inside as well if you don't see the white build up.
Another myth but i have seen it actually happen is using fully synthetic oil in high milage engines is a bad idea. The synthetic oils will cause new leaks or cause small leaks to be bigger or cause sludge to break up which causes the other two. If you do suspect its a gasket leak you may be able to buy some time with the liquid additives but not long.
All this may help or it may be a dead end. Diagnosing takes a bit of time and experience. There are other test to do for head gasket leaks do some online searches and you can find them but most of the time if it clucks like a duck and walks like duck it most likely a duck. Good luck hope you get it figured out.
