You need 8.5 qts @ approx. $10/qt. Also, you'll need to purchase an A/T dip stick-approx.$15. (It's a dip stick you only use when changing the "fluid for life" fluid.)
From what I've heard there is a process you must follow which includes heating the fluid before filling.
I'm having my mech. do it for about $200 incl. a new A/T filter.
I know I can order an owners manual for my '99 from MBUSA. The '98 and '99 cars are essentially the same.
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Admit it, we all have a little bit of Eric Cartman in us.
If you replace the filter (I would), you will have to replace the gasket to the pan. Make sure you get the correct torque spec and sequence when retightening or it will leak.
Use the correct AT fluid for this model or you will damage the transmission!