(Island, Mobile) I tend to get my oil from oily rocks up in the north, snowy, area on the very west side where there are icy islands in the water. The oily rocks are all over the icy islands! Hope this was helpful :)
if you can’t find a critter that drops oil, i found quite a lot of oil rocks in the northern region (mobile, island map), around the shore. be weary of wolves and argies. rexes too, but typically they’re preoccupied with mammoths or rhinos. hope this helped!