Really like my I30 CRDX but it's had a notchy 2nd gear slot since new. Working thru dealer and Hyundai I replaced the gear oil with VRMX80 but no go.
The dealer offered replacing with Subaru synthetic $170 plus labour, so I opted for Nulon $60 plus their $12 easy gear additiive and this helped. The dealer also tightened gear lever linkages to help.

Q) On reflection I'm now worried about my gear box warranty using Nulon . If so, what synthetc grear oils are OK ?
Red-Line MT-90 75W90 GL gear oil rates real good , so does Castrol Syntrans 75W/85. Happy to replace. Any suggestions ?

 

for your gear oil: use the recommended oil weight ONLY as well as the recommended API GL rating. ie. GL-4 or GL-5, they are very different do not mix! as long as you do that then they can't get you for warranty.

DO NOT USE ADDITIVES UNLESS THE OWNERS MANUAL INDICATES TO!!!

if you're worried about your warranty, follow the owner's manual. use only what is specified. synthetic is ok, but use what is recommended for weight/type.

as a side note, some of hyundai's manual transmissions are known to be a bit 'notchy' at times. they're solid and work well, but i know even my accent trans can be a bit of an *** at times.

 

Thanks for your advice Zero G - will select apppropriate gear oil and replace it in days. Also, think I'll drop into my dealer for a chat to be sure. Dennis Down Under