BMW Repair: BMW, computer rights, bmw 528i

Question
few days ago i bought a BMW 528i e39, the car is in the best condition but suddenly i discovered that on board computer rights before starting the car: "SLOW! CAT. OVERHEAT" could you explain what does this mean? and if you know what kind of transmision oil I can use and how can I guess when it's time to change

Answer
Second one is the easy one:  Don't change transmission oil.  It's lifetime. changing is pretty much agreed to create more problems, and is expensive and not recommended by BMW

The Cat overheating is one I haven't heard of.  Sounds like you may have a bad catalytic converter.  Getting an OBDII reading should tell you this too.  I would call a BMW dealership and ask about it.