I had this same issue on my car after installing a Pan-Baffle system. Cure was:
1. Remove pan and gasket
2. Clean all bolts, and bolt holes in pan/block with Q-Tips and alcohol/Laquer thinner-(quick dry)/Acetone/Etc.
3. Wipe both pan and block surfaces with an above cleaner, making sure any sealers-such as whats used on the 4 corners-is gone.
4. Wash oil pan gasket with Dish-Soap and WARM water to cut all oil from it. allow to air dry...may help along with cotton towel-(but wipe fibers off when done)
5. Check entire gasket for flat spots, missing peices, etc. and replace if necessary.
6. purchase some Red, high temp RTV silicon sealer, and 4 6" or longer 1/4-20 bolts.
7. place a small drop of RTV on each corner of engine block, where front and rear covers contact block...then spread this drop out thinly in a 1"x1" square.
8. thread the 4 6"+ bolts into pan corners, and allow pan and gasket to hang suspended on the heads of the bolts.
9. AGAIN, make sure Block surface is 100% free of oil drops!
10. Raise pan on 4 Bolt "Guides", and install pan bolts. Tourque bolts from center out, 95 Inch Pounds.
(the guides serve to help keep the RTV on the corners from smearing durring the typical "slide-around" install)
If leaking persists after this,you have either a porous block, porous/warped pan, or a bad gasket.