Don't do it.
They won't sludge up, but they aren't compatible either. There is nothing that is spec'd for merc V that can safely use LV and vice versa. If this is an automatic trans and you already did it a little bit won't hurt but if you put in more than half a quart you need to get it flushed ASAP. Not drop pan and refill, not drain and refill, actually flushed, total fluid replacement.