Yeah, porosity could have different meaning. I think mostly its small holes or cavities randomly spaced and sometimes they touch and can have a path that doesn't seal a pressurized area. Most holes don't have to be round and are more a function of the "out gassing" and thicker areas molten metal cooling and shrinking away. Usually thin areas near to thicker areas are more problematic for this like in the heads. I think the block is thick enough that it is more rare for a problem to occur.
It would be really bad if it was like an open cell foam that seeped out oil. Cast iron is "grainy/sandy" but should be alloyed and the material thick enough to make it is solid "closed cell form".
Now the texture of cast will have a lot of surface area if you looked at the surface metrology its bumpy grainy rough and why it will look oily and hard to clean from the tiniest of oil contamination. You can't really polish it or get it to a nice smooth shiny surface of like a tool steel or something alloyed with chromium sp?. But again its still closed cell crystalline structured just not like more alloyed steel that has a much finer crystalline grain structure. I can't remember specifics maybe the "oxygen wash" or what ever it is that turns iron to steel by stripping the excess carbon frees up the crystalline structure so it more uniformly forms ????
