A lot of paint spills will be softened dramatically with a wallpaper steamer if you have one, then just scrape off with a scraper.
Oil stains can be removed to some extent with a strong dose of washing liquid (washing not washing up).
I've used both those options before myself. Always worth starting with simple things you may already have.
Oil you can get biological agents, basically bacteria that will "eat" the old oil. Thats your ultimate.
As others have said you really want to paint the floor if these sorts of things become an issue to you. More than ever prep will be critical if you do though, poor prep on a garage floor almost always results in the patches that just dont stick and end up with peeled areas over time.