Just purchased a magnetic drain nut from NRG for my engine oil pan.

A plain ol flat washer is included..

My question is, do I use OEM crush washers instead or will the one included seal fine?

 

If the flat washer is a soft metal, i.e. copper or aluminum, you're fine, although you won't want to reuse it after your next oil change. Otherwise, the oem crush washers, again, used only once, will work fine. If you try a flat steel washer, you run the risk of overtightening the drain plug and stripping the threads, or it will just leak.