Ablecain......... Go to Troy-Bilt website
http://www.troybilt.com/webapp/wcs/...&cm_mmc=Yahoo-_-Parts-_-Troy-Bilt-_-troy+bilt
Early '70 and '80 Models did not have model numbers so were either "Pony" or "Horse" models. If you type one of those in for Model number you can get a list pulled up at the Troy-Bilt website and match it up. Or call their customer service number and give them your serial number and they can match it up for you. You can download Owners and Parts Manuals from the website.
The other answer is correct with SAE 140 Gear Oil for hot weather, and SAE 90 Gear Oil for colder weather.
I had a Troy-Bilt '72 6hp Horse that I got from my parents and used for many years, and I think it's still running, ex-wife got it. Just this last weekend picked up a Troy-Bilt '87 8hp PTO Horse.
I can't speak for the new ('92 and later), Troy-Bilt was bought out by MTD, but the old Troy-Bilt's can't be beat. My dad is still running a '74 7hp Horse.
Good Luck