NissanRWD

Edit: Car is Manual Transmission, my bad

I bought the car at 124k and it seemed well taken care of and had a coolant flush and refill along with oil changes on time when I bought it. The car had no shift boot and often i'd smell a little bit of burning transmission fluid but only on hot days and when the engine was warm (fan clutch kicks in).

Idk when the last transmission oil change was done but I want to do a flush. I've heard ppl say that flushing a car with high miles and not knowing when the last change was done can possibly ruin the transmission by taking out all the crap that's built up or something. Can anyone explain this to me? Maybe give me an idea on what I should do?

Transmission shifts fine btw. Sometimes has a click to it but doesn't necessarily feel like a grind