As its a manual I doubt the oil is the cause of it slipping, that sounds more like the clutch to me.
If it was an automatic then it should have the Kia brand auto trans fluid available from a Kia dealer
Apoloil Red-1 ATF although some people have used Motul Multi ATF and Castrol Transmax E
The transfer box should have <a>1.5 L of ATF (Dexron III) or equivalent. </a>
<a>STANDARD FRONT DIFF OIL - 1.5 L of SAE90 Gear Oil (GL5) or equivalent. </a>
<a>STANDARD DIFF OIL - 2 L of 85w-90 Gear Oil (Limited Slip) or equivalent.</a>
Hi, I know this an old thread but did you ever find out what this was? Mine is doing the exact same thing and, my mechanic hasn't got a clue. Have had the transmission fluid replaced and new fuel filter but still doing it