Oil is NOT just oil. There are many additives and base oil types. The engineers that designed your tractor used certain oil properties when specifying bearings, gears, friction plates and such. Only they know the specs. Industrial equipment usually recommends certain oils by name for that reason. Except for oils with the API mark and grading, and so specified by the machine manufacturer, I only use manufacture's oil. Deere, CAT, Kubota, et al. don't produce oil but they do specify oil properties and minimum requirements. The oil producer meets those specs or faces huge financial liabilities... not only loss of a customer.
Consider: when the oil container says "recommended for ..." Ask yourself, who made the recommendation ? Or when they say meets xxx specification, who verifies that it does ?
Oil is cheap compared to parts and labor for failed equipment.