Toyota Repair: 19914 RUNNER mysterious oil leak, freeze plugs, intake manifold

Question
Chasing down an oil leak on a 91 v6 engine I removed the air chamber and intake manifold and found aproximatly a half quart of oil under the intake leaking off the back of the engine. Cleaned and inspected top of block but could'nt find a crack or any evidence of where the oil is coming from, any ideas. Toyota rebuilt the engine at approx. 135k, it now has 170k. It is a fully loaded 4 Runner 4 wheel drive and automatic

Answer
There are three freeze plugs and the coolant transfer pipe under the manifold, these can leak coolant but not oil, I have no idea how oil could get into the valley below the intake manifold exept maybe a leaking valve cover but even then the oil would have to get past the intake manifold, I just don't see how that is possible, mever seen oil in there, sorry can't be of much help on this one.