80 hrs OT/month is only 10 extra hours per week or 2 extra hrs per day. You'll find businesses where there's an unwritten rule that for engineers, 40 hrs really means 50, so this isn't all that unusual.
Is it "right"? Is it "over-work"? Depends on you- and on your compensation scheme.
If you're not getting a piece of the action, either via meaningful profit sharing, ownership, time in lieu or overtime pay, working consistently large chunks of O/T for free is an indication that you need your head examined. Why exactly are you doing it? What hole is there in your life that work is filling, to the point that you're willing to do it totally voluntarily? I suggest that there are a great many REAL charities which would truly benefit from your volunteerism- why volunteer for a profitable corporation?! Think of the people that businesses aren't employing because others are willing to work for free...
There's a difference between being a professional and being a sucker. A professional will put in some extra effort to see a project meet a deadline, or to learn something, or to fix their own mistakes. A professional also expects that their contribution will be respected, monetarily, or else it will surely be taken for granted and indeed later it will be expected!