I bought a refurbished MacBook Pro a few years ago. It started to run slow recently so I want to buy larger RAM. I realized only one memory slot is filled, with an 8GB. When checking for compatibility, my processor isn't listed. Would this matter when purchasing new RAM? Also, if the Apple support page lists max 8GB, does that mean I can have two 8GB, or have to have two 4GB? Thanks for any help.