Old stuff leaks...
Motor oil and diesel fuel.....you should be able to tell them apart in the bilge. Anything or thing(s) can leak and it's hard to tell from here. My favorite method is a roll of paper towels and wipe things down clean and then go back a little later and look for fresh drip/puddle. Also, Lay some clean towels down in places you can't reach and then after a bit of a wait pull them out and check for stains.