Here is the deal, when I first dismantled the Gas Pistons, I saw that there was some grease on both of the O-Rings. Not on the pistons, on the rubber O-Rings.
My M4 Supplier (merchant), said NOT to lubricate/grease the tubes OR the O-Rings at all.
The manual says:
However there is no word on the O-Rings which from factory had grease. Shall I apply grease on the rings or not?