Just drained the "life time" fluid from my car (2007 with about 30k miles) and the fluid was very very dark. I am sure it might be ok but i am glad i changed it and will do it at 30k from now on. I brought a container to the local deal and got the OEM liquid gold (at $35/litter
I did the Rear dif earlier and the old fluid wasn't too bad. I replaced it with AMSOIL.