Because the hydraulics are supposed to have their oil changed at 1000 hrs , the same interval as the chain case, I usually drain and clean the chain case , then drain the hyd reservoir into the chain case then refill the hyd and top up the chain case with fresh 10/30 motor oil. I don't know if there is any harm in recycling the hyd oil into the chain case, but its filtered and clean, but I can't see the chain oil wearing out, but it is good to remove the wear from the chain case periodically. My $.02 KenPut the machine on level ground, drain the old oil and with the brake assembly cover off, mop the remaining oil from the bottom of the chain case with rags. This is done to get rid of any metal particles from wear of the chains and sprockets, you will see black sludge on the bottom, this is what you must get rid of.
As for oil, use standard 10w30 or 10w40, this is the same oil as your hydraulic/hydrostatic system uses, or you can simply use normal engine oil. Fill it so its just starting to come out of the front hole, on a 7 series loader thats about 35 litres, just for an idea of how much oil you will need.
As far as i'm aware, the reason they want you to dump your hydro oil overy 1000 hours is because it absorbes small amounts of water and starts to break down, but don't quote me on that. I honestly can't see a problem using it in the chain case, as ken said, its all filtered. I have done it myself on my 731, as it was always leaking oil so i was always topping it up. When i changed the chain case oil i just filled the tank and moved around a bit then opened up an aux hydraulic line and squirted it into the chain case, i did that a few times untill the chain case was full.Tazza and Ken Thanks alot for the info! Very helpfull,and good idea on the recycled oil Ken!Thanks again for the time and effort you guys put into this fourm! Randy