The old hands will always tell you that when an autobox is playing up, first change the ATF fluid - it may or may not correct the fault but will eliminate dirty/contaminated ATF as the cause - it''s relatively cheap for most cars.

you must use the hyundai/kia reccomended transmission oil.

Like all automatics, the correct ATF MUST be used - in this case Hyundai SP-III but there will be Hyundai, Kia and Mitsubishi versions of it as well as good reputable third party products which are compatible - eg Fuchs Titan ATF 4400.

Hyundai SP-III is "just" a good Dexron 3 - but not all Dexron 3 is compatible with SP-III