Sounds like a heavy case of sludge, yuck! I would suggest a one of a couple of ideas. First option would be to do a flush on the engine. The forum recommends, as well as myself, using the Seafoam engine treatment. This would be something that works pretty fast and could fix the problem in just a few minutes. Just take a can and poor it into your crank case and run the car. Just keep in mind that you don't want to drive the car since this will likely reduce the viscosityof your oil, and then change the oil. I usually just run the car in the driveway so that I can monitor what it'* doing. Some people think that doing such a quick flush can do more harm than good since it can loosen larger chunks of sludge and stop up the oil pump, so it is a bit of a gamble. This would lead to option 2. Look into Auto-Rx here Auto-Rx Engine Cleaner Transmissions Motorcycles Stop Leaky Seals Oil Burning Increase MPG Better Gas Mileage Trouble Shifting ARX
It works at a much slower, safer, rate but also does a great job cleaning the engine. I used it on a car that had a leaky rear main about a year ago and it had all but stopped the leak by the time I sold the car. Kinda pricy but worth considering. I currently have some in my 95 silverado with lots of miles on it. I hope this gives you some good ideas, but I think a good (or several) engine flush is what you need.