they call them durapro which is now discontinued.
"M" is the more durable chain I think you're thinking of..... The M is for "multicut" which is for extremely dirty, abrasive cutting conditions, as said in the Oregon catalog.
I thought the x had the little extra hump for the raker from the link in front for smother cutting?
P vs. G as stated above
If you guys want the rundown on different Oregon chains I'll do my best here to get it out of the Oregon catalog so you can see it in their words.
LG is no longer in the book. Not a low kickback chain. Square corner, round ground 3/8 chain. Ramped depth gauge.
LGX - Round ground chisel cutters with ramped depth gauges and standard sequence. Patented steel, OCS-01 steel. Blued cutters provide superior corrosion resistance and improved strength. "X" grind - enhanced grind geometry significatly improves out-of-box sharpness with substantial speed improvement in boring and bias (undercut) modes. Offset ramp-shaped depth gauges reduce roughness, helpos prevent cutter from diving into the sidewall of the cut. Clean ground gullets, increase chip flow and evacuation.
LPX - Description is same as LGX except the depth gauges are a bit smaller. They also employ a semi-bumper drive link with each cutter. This chain is listed as "not a low-kickback" chain as the bumpers don't particularly interfere with the cutting, but the purpose they serve is to create a low vibration chain for the user.
M - (MLGX, MLPX) Specially built chain with extra-heavy-duty chrome on the cutters for effective cutting in extremely dirty or abrasive conditions. (Note: M is used as a prefix)