I read that the T520 can handle 16BG of RAM despite the fact the Lenovo's configuration shows that you can only have a maximum of 8GB.
It will depend on the exact processor model you have installed because ever since the Intel i series mobile CPU's it's them and not the north-bridge chipset that determines your system's memory capabilities. Generally speaking 16GB is possible and you're best bet is PC3-10600 DDR3 in the form of 2x 8GB SODIMM's as I doubt the T520 has four RAM slots for a 4x4GB config. Intel i3 CPU --> What laptop memory for my Intel i3 mobile processor powered system Intel i5 CPU --> What laptop memory for my Intel i5 mobile processor powered system Intel i7 CPU --> What laptop memory for my Intel i7 mobile processor powered system
That proc supports 32GB of RAM but the T520 only has two SoDIMM slots so your max is 16GB (currently). See Intel Processor Specifications