Marvel mystery oil will do. Is it stuck or will it turn? Very few engines like to sit 6 years not just big blocks. I would drain the coolant, flush the radiator and change the oil and filter first. This keeps any crud that has settled over the years from circulating through the engine. Get it oiling first by turning it over without the spark plugs in short burst, I was told 5 or 6 short burst over a few hours gives the oil time to get where it needs to be. Use good gas not 6 year old gas. Put the plugs in then fire it up. The chances are that if it has been in the garage it will go fine. Then you may have to pull it and rebuild it if it turns to crap.
