To break something like the oil pan loose I usually place a piece of 2 X 4 against the pan and hit the wood with a heavy hammer. Start with light blows and get a little stronger as you go, and move from side to side. I also have a set of nylon wedges which I use to open the seam. You can get inexpensive tools at an auto parts store that are used to remove interior paneling and that may also help get the seam opened. You can cut the 2 x 4 into a wedge (hard wood would be better) and try using that to get the seam to open.