I did the 2.5 to 2.2 swap. The 2.2 was sitting on the oil pan when I picked it up and I (unfortunately) did not think much about it... however, previous owner said it had 75k and he replaced rear main seal with the newer one at 50k when he got the engine, so I was not worried about that.

 

The leak is coming from the back of the pan/engine, but its hard to see back in there. I assume that the engine resting on the oil pan was probably very bad for the seal and hope that's where the leak is coming from.

 

I don't see anything coming from higher up, so seeing oil at the back of the pan is either the oil pan itself or the main seal right?

The oil separator plate is also VERY prone to leaking. That is located back there as well. Before you put it in was it wet back there? Was the pan leaking when you had it out? When I have the engine out and if the pan shows any signs I leaking I always reseal. You can reseal while its in but it is a pain.

Nothing was wet back there when I put it in... everything looked fine and then a slow leak started after a week or 2 of driving. It's still a slow leak today (6 months later) stop leak seems to help, but its slow so i cant tell definitely.

Also, I double checked and got it backwards, the guy I bought it from said he replaced the oil separator plate but not the main seal.

With what you are saying, typically a slow leak from the back is a rear main seal unless you can pinpoint a went seal around the pan.