From Chilton Repair Manual, Volvo, '70 - '89....

"Sparkplug Diagnosis...Oil Deposits...Appearance: The firing end of the plug is covered with a wet, oily coating. Cause: The problem is poor oil control. On high mileage engines, oil is leaking past the rings or valve guides into the combustion chamber. A common cause is also a plugged PCV valve, and a ruptured fuel pump diaphragm can also cause this condition. Oil fouled plugs such as these [pictured] are often found in new or recently overhauled engines, before normal oil control is achieved, and can be cleaned and reinstalled. Reccomendation: A hotter spark plug may temporarily relieve the problem, but the engine is probably in need of work."

pray its not the rings. of course, there are many factors not mentioned that are importaint to decifering what is going on. and btw, you can't usually turn the crankshaft with just the fan belt, unless you take the sparkplugs out or something first. If you could, it would probably mean really poor compression.
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-Ben- '71 142S (parts), '71 144S, '74 164E