shifting,Gear shift slips over to Manual Transmission without touching. I have to hold it to keep it in Automatic.

Gear shift indicator can either be Automatic or Manual – when putting in "Drive" from the park position – it slips over to Manual Transmission – I can push the knob and hold it in place ( for automatic) but that's not an optimal way to drive — what gives ? Also – does this model require transmission fluid? I was going to check that – can't find in the owners manual where that would be … please advise

My car has 180000 miles.
My car has an automatic transmission.