I've never had to jump through hoops to get warranty work done on a Hyundai vehicle. Any carmaker will require the dealer to check the condition of the fluid before approving any warranty work on a transmission. If the fluid condition looks bad or even borderline, it opens the door for more questions such as "when and where did you have the fluid changed."
If a multi-vehicle ATF is used which lists the required spec (such as SP-III), then there is no arguement from the carmaker regarding the fluid used, since the fluid was tested and approved by its manufacturer for the spec required by the particular vehicle in question. On the other hand, if the transmission was serviced at a quickie lube that uses non-spec fluids and a friction-modifier-in-a-bottle which supposedly converts the fluid to other specs, there could be some difficulties in getting the work covered under warranty. Hyundai (along with other carmakers) specifically states in the owner's manual not to use engine/transmission addives in the owner's manual.