I know of a 1966 John Deere 4020 PowerShift with over 16,000 one-owner hours. It spent the first 9 years of it's life as the #1 tractor on a large dairy farm. In 1975 they added a 4430. The 4020 never had to do primary tillage work again, but was used to pull a planter, cultivate, cut, rake, and bale hay, run a silage blower, and in general, never resting. By the late 80's, a 4850 was added to the fleet. The 4430 took on a lot of the 4020's heavier chores. The 4020 got a loader installed and spent it's years since as a barnyard tractor, and with a hay grapple, picking up baled hay and straw. It looks the part of a 16,000 hour farm tractor. The engine was overhauled somewhere around 10,000 hours. The PowerShift tranny, other than oil changes, has never been touched.
Also, I have my fathers 1956 Ferguson F-40. It has logged over 15,000 hours before the hour meter quit. That was in (approx) 1990.