Oil and fuel would look different in the bilge. Oil can leak from your transmission, or the engine main seals (front or rear) and be hard to trace. One thing that can help is to put a "diaper", an oilzorb pad, under the engine so any drips or leaks get caught before they run further. To find the leak you'll need to start by cleaning the engine and engine bay. Then place a fresh liner under the engine.
At that point you can spray talcum powder on the engine (it won't hurt anything) and after you run the engine for a while, look it over to see if there is anyplace that there are oil stains running through the talc and dripping under the engine. If you still can't see anything, you can put a UV tracer dye in the oil, run the engine a couple of hours, and use a black light (same kind sold for tracing AC leaks but any strong black light would work) to see where the glowing traces from the dye and oil are.
Residue under the tank could be fuel...but after 30 years it is time to pull and check the tank. It might be a monel tank--top quality and very expensive. Even aluminum can last longer than that without problems, but if it has pinholed, those can also be welded (after the tank is emptied and cleaned).
On a thirty year old boat? Odds are everything leaks.