[kaiser]

1. The oil cooler can leak internally or externally. An internal leak you can only see by water loss, and if large by cross contamination.
a small amount of water leaking into the oil will "burn off" and be invisible.
Only if too much will it be visible.
2. The inlet manifold gaskets leak, irrespective of how well they have been fastened. I have yet to see a set of gaskets that have not leaked.
The amount might not be great, but then again it could be. Difficult to tell before you see the gaskets. If you have no deposits on the spark plugs. I would try and by-pass the cooler as a first step.

This is a matter of trial and error. So complete one thing at a time and give it time to play out.