Wayne:
When most hear the word porosity in relation to head or block castings, they automatically think of the water jacket. Porosity can occur anywhere in the block including anywhere the
oil is passing through.
If this is the problem, it may be difficult to locate. You can have a pressure test done just like you would for water, but there are many passages to block off. The block would need to stripped down to nothing.
Dove is very well known for casting problems mainly of which are metal that is too soft, and porosity. You may want to call George at Guessford, he was dealt with Dove for many years.