I`d be a bit carefull adding syntetic oil on a engine been running on mineraloil for a lifetime.
syntetic oil have better cleaning/dissolving properties and can cause problems since by cleaning to mutch too fast and clog up oilcannels and filters.
So if u want to run it, wash the engine w diesel first, Just empty it and add 3\4 w diesel and rest oil and run it for a couple of minutes.
Happy new year from Norway
