If it's running ok othewise I'd add a quart of engine flush, marvel mystery oil, quart of kerosene, something to flush out the system, thats the quick way. A slower way is to put a 50/50 mix of regular oil and synthetic oil. The syn will break the sludge down and should clean the crud out, just takes longer. Also keep an eye on your dip stick, the syn and the cleaner will get the oil dirty real quick so you need to change the filter as soon as you can no longer read the hash marks or writing on the dipstick, otherwise the bypass will kick in and you will be pushing dirty oil thru the engine.
Also you may see some leaks start to develop, thats the cleaners cleaning the sludge that has built up around the gaskets and allowed the gaskets to dry out and more or less dry rot. Thats a seperate issue.