4GB Memory RAM for Lenovo ThinkPad W Series W500, W500 (4061B15), W500 (4061B13), W520, W700 204pin PC3-8500 1066MHz DDR3 SO-DIMM Black Diamond Memory Module Upgrade System Specific Memory - Newegg.com
My Thinkpad W520 despite some clocking issues while on battery. I upgraded it to 16gb ram : r/thinkpad
CMS 16GB (2X8GB) DDR3 10600 1333MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with Lenovo® Thinkpad W520 Quad Core 4270; 4276, 4282, 4284 - A13 at Amazon.com