Kind of rare (I've only had it happen once myself). Those senders are all cheaply made and crimped together with some sort of internal gasket. Mine was squirting pressurized oil out the riveted-on lug! I would throw it away & replace with new but if you're obsessive about components, hose it down with rubbing alcohol, then when its dry, sprinkle it with a small amount of flour. Start the engine and run it only for a minute. A close look will tell you where the oil is coming from.
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