Does anybody know where this leak might be coming from

Oil cooler o-ring.

might be the gasket around the oil pan. clean off all the old oil and drive it for a day and check again. you will get a better idea where it is coming from.

^what he said.

it's an O ring between the metal thing where oil filter screws in to and the engine block.
What you need to do is
1. Drain the oil
2. remove oil filter
3. remove oil cooler
4. replace O ring
5. Put oil cooler back
6. Put new oil filter on
7. Put fresh oil in.
NO LEAK.

Nissan Quest procedure. G35 is very similar.