Burns did't write Auld land Syne, but collected it (his own observation). There are many regional dialects and pronunciations in Scotland and coming from the West coast, I have always used 'syne' although I doubt if it's used nowadays in everyday speech. My father and that generation used 'sinsyne' a lot, meaning 'since then'. I would think that a Scot in England who says 'sin' is being merely lazy (or is very old)!