CMS 8GB (1X8GB) DDR3 10600 1333MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with Lenovo® Thinkpad T420 4177, 4178, 4180, 4236 Series - A14 at Amazon.com
8GB Stick for IBM Lenovo ThinkPad T Series T420 T420i T420s T520 T520i. SO-DIMM DDR3 Non-ECC PC3-10600 1333MHz RAM Memory. Genuine A-Tech Brand. at Amazon.com
Lenovo ThinkPad T420 Review - Itechguides.com
Finally upgraded the RAM in my X220 : r/thinkpad
Memory for a Lenovo - ThinkPad T420 - Kingston Technology