Isuma

I bought it n 2015 and I used about 30k . I think now it is due time to oil replacement. Oil replacement should be done by 40k right?

I am very new to cars. I have a vitz 2009 model and its mileage 80000 Km now. When I drive I can feel the gear changing automatically, so I think its time to a gear box repair. So I need to know whether 80K is the normal gear box repair interval or else. If I change gear box oil and wait until 100K? And how much cost to have a gear box repair?