I just got my second M305 last week... Use break free "CLP" and clean everything.
Open up the gas plug "after disassembly" and use Q-tips and clean the inside of the cylinder, the piston... Everything. Use some CLP on the piston to clean all the packing oil off... But then make sure it's dry... Both the piston and the internal area of travel.. Fir the piston... "the plug side" Both sections. Try and take the piston out... But if its a shorty like myne...The bayonet lug won't let you. Do the best you can. In the end the piston needs to be clean and dry. If you wipe the rest of the rifle down with CLP... Then dry it off with The cheap colored microfiber cloths from Canadian tire... You will be surprised at how nice it cleans up.
Follow the greasing instructions above.
"I went with white lithium bearing grease"
Good advice here

Getting all that chinese packing oil/cosmoline/grit out of all the parts. A lot of guys break out a tooth brush and a lint free rag and go town on 'everything" with Simply Green.
I like to use sunlight dish liquid cuts grease like a mudderfugger. I use a 45 acp soft bore swab for cleaning gas cylinder.
Once all the grease, oil it came with is removed, CLP everything and wipe down. Lordco has Seafoam deep creep in a can and same as CLP.
Bolt (inside), Gas system, cylinder and piston run dry, no lubricants. I personally do not apply grease or anti sieze to gas plug... That's a new one for me... Don't recommend this at all.
For you shorty owners having issues getting pistons out ...
1)Remove gas plug
2)Undo and unthread gas lock, slide towards flash hider
3)Slide gas cylinder towards flash hider and rotate so piston can be removed.
Sometimes all ya gotta do is undo the gas lock, rotate it out of pistons way and it will come out.
Last piece of advice... Always make sure that piston is sticking out of the oprod end of the cylinder before tightening down the gas plug (90 inch pounds torque)
Grease her up, run a patch thru bore, verify safety checks and feed her some ammo
