Long time to Reverse
 

The car takes about 5 min to get into reverse when the tranny is cold even when its warm it somtimes doesn't want to reverse. I tried changing the atf and putting some lucas oil additive. Is there any additive known to fix this problem? Or anything else I could try.

Because it does go into reverse, I'd say it's probably NOT a major mechanical failure. My knowledge or 928 auto is limited, but from general auto trans know-how: Did you change the filter? A very dirty one would have an affect on pressures, or an adjustment may be needed; bands, modulator, throttle cable?
Sorry can't be more helpful than that.

Ya I did change the filter the old one had a lot of black junk in it.

If the filter was full of clutch plate material, ie dirt. the trans is coming apart inside. It takes twice the pump pressure to go into reverse than into a forward gear. It the dirt in the pan hade alot of metal shavings in it the torque converter is going south. It sounds like the trans is worn out and can not build up the correct presures. If you can run a presure test on it you will find the problem. It is listed it the factory manual. Or do a search on the web, this is a mercedes benz trans. Transtar has a repair manual for this one

Good luck