Hyundai specifies manual trans (gear oil) with a "GL4" rating.... weight being 75/85. and it is a synthetic blend.
DO NOT use gear oil with "GL5" rating... the chemical package will ruin the brass used in the synchro rings.
Correct oil is available at Hyundai and Kia dealer, part number is 08950-00020,,, be advised, it is kinda pricey at $15/qt I think it is..
You might be able to find something on the open market, we used to use Penzoil out of 5 gallon pail at last dealer I was at, and the Hyundai stuff on warranty tickets.
Easiest way to fill is to remove the reverse light switch on top, use long funnel and fill through top..... be sure to remove fill plug on side of trans case so when oil reach correct level, it'll run out the fill hole to show you at correct level,,, install fill plug, then re-install switch.