These are equivalent to NGK CM6 spark plug:

NGK STK: 5812
AC Delco: M8 (Non-compact size, but similar thread and thermal rating)
Champion P/N: UY6
Champion STK: 842
Denso P/N: U20M-U
Denso STK: 6003
GM OEM NR: 1559459
Motorcraft P/N: BT8
Napa: X581
Prestolite: 09-10-06

http://www.sparkplug-crossreference.com/index.php

NGK numbering system:

http://www.ngk.com/sparkplug411_manu...nufacturerID=1

The C indicates the M10 thread and 5/8" hex size.
The M indicates a compact size.
The #6 indicates a medium heat rating (2 is hottest and 12 is coolest).