You do not have to use Dell RAM and it will not void the warranty. The only consideration is whether the new RAM will be compatible with the existing RAM. The most compatible RAM for Dell systems is Crucial as found here. The ideal solution would be to buy a 32GB kit (2 x 16GB) which would be a matched pair.
Hello. I’ve just bought an Inspiron 3020S. Very happy with it. But I’d like to increase the RAM from a 16GB (single)in DIMM 1 to 32GB by buying another 16GB to add to the spare DIMM2 slot. Must I use Dell memory to be compatible with what’s there already and maintain warranty? It’s far more expensive than other suppliers for example. But if otherwise it voids the warranty then maybe I’d need to pay up?  Also with the new memory being inserted the CPU would be accessing dual channel. Will the Dell memory be matched to the existing memory - Speed , latency etc… ?  Or would I need to get a matched kit? 
Memory Upgrade in New Inspiron Desktop