I ordered my Retina iMac with 16gb of ram from Apple. I know of the talk about matching ram sticks and all that, so I'm wondering with that in mind, if wanting to one day bump up to 32gb of ram (adding another 2 8gb sticks), can I just order 2 x 8gb sticks from somewhere, or would it be that I need to purchase a fresh set of 4 x 8gb sticks? With OWC, Crucial and Kingston being the forerunners of Mac memory, do any of these actually make the exact same ram that's installed in Retina iMacs? I don't mean the manufacturer, I mean running the exact same tech specs which Apple recommends/uses. I read a few articles that OWC is slightly different or Crucial is a little different. Thanks all.