There has been a lot of debate on this forum and other forums about the wisdom of just changing filters and not the oil.
Without restarting the whole debate, the 50-hour service is the first and most critical. The oil is full of manufacturing and assembly debris. A filter takes out most, but not all, of those metal shards and metal powders that are created as the engine, bearings, and wear surfaces mate.
For the price of the tractor, and the relative inexpensive oil, I drain/change the oil each time it is scheduled to change the filters. Tractor transmissions are subject to tremendous stresses on occassion.