Where did you see that Thinkpad X220 could andle overclock memory?( please Give link)
Lenovo spec ore
8GB max7 / PC3-10600 1333MHz DDR3, non-parity,
dual-channel capable, two 204-pin SO-DIMM sockets
But if your laptop could run OC memory they will be memory option in bios
Intel extrem edition motherboard run OC memory with a XMP option in bios
I don't now if you have this option in your laptop bios or even if you have the option to OC memory.
A other option to reach 1866 MHz on memory is to OC cpu
Memory will automaticly OC as CPU increase speed.
Cheak if ther is a bios update
As it improve compatibility and stability ove system
Hope this help