Its not like engine oil.It doesnt get fouled with combustion debris,and doesnt get as hot either (normally).Hydraulic oil doesnt "wear out",it can become contaminated with solid particles from excessive wear or impending component failure.The biggest killer is heat,which causes it to oxidize and varnish components.Control the heat,and in theory,the fluid should last forever.I did a complete fluid change on my 4r100 when it was new,about 20 miles and switched it to Amsoil.Never touched it fot 110k miles,and when I did, the fluid was just like it was when it went in,no scum in the pan.Put it all back together with a pan fluid only change and ran it 8 more years untill this spring at 200k.It was working fine except it had developed a significant flare going into OD,so I just changed the whole unit out for a BD.I did ,and still do have a magnetic inline filter that I change every year,but the fluid in the old one looked and smelled fine at 200k.Understand that I have run and towed the crap out of this truck since new,and that hasnt changed.Even pulling the 5er I cant get the trans over 180 degrees,I dont know about the 4r100 because I never had a temp gage on it,but the oil never turned color or stunk.I think if you can keep it below 210 it should run forever,barring mechanical failure,on the initial oil fill.just my 02 and experience on the truck but the 04 Grand Cherokee is basically the same story with 190k on it.
