I'd pick over https://www.google.com/search?q=e6520+compatible+CPUs&oq=e6520+compatible+CPUs and see what folk are reporting as compatible.
Tip: Be sure the BIOS is current before any changes.
I just bought a used e6520. it currently has a i7-2640m cpu installed. I have upgraded to a ssd and 16gb of memory. I want to know what is the fastest cpu this laptop will support. Any suggestions would be greatly appreciate it