Manchesi, if it's slipping, don't even think about changing the fluid, or you stand a good chance of making it worse, or killing it completely. If it's still driveable, swing by the shop and I'll run a diag on it to see whether it's the transmission itself or one of the control mechanisms (sensor, TCM, loose wire, etc.), and help figure out a course of action.

And yes, it takes standard Dex III (standard service) or Shell LA2634 (lifetime service).

address & phone number in my sig.