RAM upgrade MacBook pro 15 inch mid-2010 on "El capitan"
Officially your Model only supports up to maximum of 8 GB RAM. So you are at maximum already. 

http://www.everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i5-2.4-aluminum-15-mid-2010-unibody-specs.html

Your alternative is to upgrade from Hard Drive to SSD, that would speed things up.