My grandfather was Polish, yet he never spoke the language with me. Via this course I feel like I am rediscovering a part of my family, and even of who I am.
They are in the middle. They are used in most situations both formal and informal.
Polish has many levels of diminutives, so there are more informal ways to call grandparents like dziadzio, dziadziuś, babunia
There are also old words for grandfather and grandmother, that while more formal are also less used because they are at the same time sometimes rude, and have like 8 other meanings each (dziad, baba, babka)