1995 Buick LeSabre - Engine Oil Leak

I have a 1995 Buick LeSable (3.8) that has developed an engine oil leak. The leak is a steady drip, 1 drip about every 4-5 seconds, that runs off of
the oil filter. It only leaks when the engine is running. I checked the oil filter gasket and it was fine. I suspected the oil filter adapter gasket, as the oil seems to run down the edge of that gasket, so I replaced it. The leak continues. I suspect the only other possibilities are the front crankshaft seal or timing cover gasket. Is this type of leak typical for this engine? What normally causes this leak?
Thanks, Steve
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