My moms '90 300SL is having problems with the automatic transmission. The reverse clutch is worn out so the gear doesnt catch sometimes. A friend recommended draining the ATF and putting gunk transmedic in and letting it run for about 100 miles. then draining and replacing. I see how this could help the transmission but I don't see this having any effect on the clutch. Am i correct here?

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W126 1983 300SD 286,000 miles and ticking
Baby blue exterior Grey MB tex
Recent work:
Replaced air cleaner mounting brackets and heat shields
Replaced alternator, fan and power steering belts
Replaced positive battery terminal
Replaced negative battery terminal and cord
New Duralast Battery

My car needs work.