On 2 occassions, I have had the no.5 main cap studs screwed all the way down, and still slightly protruding too high and slightly lifting windage tray preventing it and oil pan from seating properly. Both engines slightly leaked and there was very slight witness marks on the tray where contact was.
To correct, i used a bottoming tap and tapped slightly deeper to clear stud to tray.

You can see the contact witness marks on rear tip of main studs in picture.

Just a possibility to consider if you installed studs with a windage tray or girdle.