if it's leaking from the on/off on the front, you may just need to clean it, or install new orings. i personally like the cp plug because it smoothes out the front of the older dms, making them much more comfortable.
if you have an oring kit, then i'd just change the orings if you think they are suspect. videos on how to service the hpr can be found all over youtube. the front on/off is detailed in the manual.
as for lube, any of your dow 33 based greases will work. Planet Eclipse grease, sl33k, hater marmalade, lurker lube, monkey poo, slick honey, etc all work.