If you just had a little oil on the threads that may be where some was on your head from a valve cover gasket leak. If the rest of the spark plug was full of dry, black sooty stuff that is called carbon deposits. You should get another set of spark plugs and make sure that the gap is set correctly for them and then install them, and see if that helps. The Haynes manual says to make sure that they have the correct heat range. And too check for clogged air filter or problem in the fuel system and engine management system and also check for ignition system problems. Well, that doesn't help much I'm sure I know you probably wanted something more narrowed down a little.
If it was oil deposits instead you may have oil leaking past valve guides or oil leaking past your piston rings.