You don't need to add Lenovo own specific memory, as far as the memory is compatible with the X61T. Basically all well known memory vendors (Corsair, Kingston, Apacer, Hynix, Patriot ...and so on...) should have the one or other compatible SO-DIMM memory modul among their product portofolio. Many of those memory vendors do have and publish notebook compatibility lists for their memory moduls, so you should check if the specific memory modul of your choice is claimed to be compatible.
Actual modern notebook chipsets and BIOS versions usually support and allow to mix different memory modul capacities among the memory slots, but for sureness you should better look what other X61T users are reporting here, e.g. if this works fine without trouble or performance losses (keyword: dual-channel memory) for other X61T users.
ThinkPad T60/X32/600/770 · IBM IntelliStation · 3x IBM SpaceSaver II