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My 99 just developed an oil leak as well. Similar to yours. This might be dumb, but what side is the leak on? Mine is on the left if you are standing behind the car. I just had the oil changed recently and before that there was never a leak. Wondering if something else is developing.
If your spark plug tubes are 18 years old they almost certainly are leaking. Fortunately they are cheap to replace and swapping them out is fairly simple.