I found a metric magnetic oil pan drain plug for the Xterra. It has a flanged hex head, instead of the plain hex head that you normally find at the parts stores. Flanged hex head is like the factory drain plug (i.e. washer-faced flange). It also has the same amount of thread engagement as the factory plug. The magnetic drain plugs I've seen at the parts stores are sorely lacking in thread engagement depth, causing me to do some searching.

Thread size is 12mmx1.25. It takes a 14mm socket or 9/16").

I got mine from CG Enterprises, part number 1040M, for about $3.45. They're a Canadian company.

http://www.magneticdrainplugs.com/metric.htm