
+2, I'd take this risk; flush the tranny to try to get any gunk or self-consumed bits out of it and drop in a heavier oil.
It's a 7 year old car, not uncommon for it, the Optra is a GM-ized Daewoo Nubira. The Hyundai store I worked at did the service for Daewoo's when the owner had the franchise, auto trannies were a common complaint/replacement item when under warranty. How many km's do you have on it?
I can see why your GM dealer is unenthusiastic about it. Low volume, inexpensive car with oddball parts by GM's standards. I'll bet strongly that dealers have a general hate on for them.
If you really like the car and don't plan on trading, see if one of those Mr. Tranmission places will give you their opinion. You may be able to sink maybe $1500 into it and squeeze another year or two out.