Does the W520 have more than two RAM slots? I have one at work but have never cracked it open to look.
If all you're looking at is a 2x8GB kit, then just make sure to get JEDEC-spec 1.5v or 1.35v PC3-10600 and you'll be fine.
http://www.crucial.com/upgrade/Lenov...-upgrades.html
http://www.newegg.com/Product/Produc...82E16820148469