The 1 to 2 change has always been a bit of an issue on i30s.
It has been discussed many times on the forum and having owned both the earlier FD and the later GD (both diesel), I can say that the GD is better but still not great.
With practice, you will get it to be much better because it has to do with both the speed of the change and also the direction you move the gear lever in its plane, if that makes sense. The direction you pull the gear lever from 1 to neutral and neutral to 2 is not as intuitive as you might think.
Experiment with it and you can get it smooth 90%+ of the time.
It's very satisfying when you learn to do it right and it did take me some time.
Although as Dazz said, if it's leaking oil, that's a different matter that needs to be investigated.