If the oil is old it would be best to change it out. Moisture is the big enemy here.
Of course that being said, you can never get all of the oil out of the hydraulic system since it does not circulate. So different oils get mixed all the time. Especially if you rent attachments. Some people also change the weight of there oil depending on the season.
The chain case is just an oil bath. The big thing here is to keep water out of it. You should pump some of it out. If it looks milky it has water in it And should be completely changed out. Chain cases have condensation in them and a lot of water accumulates. A lot of owners actually use the old oil out of the hydraulic system to help fill the chain case.
If you are only topping it off it probably won't make much of a difference what you use. Back when the 632 was manufactured Bobcat said it was ok to use 30W engine oil. In my older machines, pre 2000's, I used NAPA brand hydraulic oil AW46. It's cheap and I never had any problems with it. The weight of the oil will depend on your specific climate.