could be two differ problems
1 to rich
2 slight oil leak
i think your looking at them as the same problem
what you describe comming from the tail is the factory tune and no cat
the oil could be from anywhere, check your oil pan it might be seeping slowly on
to your header under the block
one more thing,, it doesnt have to be comming from the engine , youur tranz takes moter oil also, give that a once over