Wipe the synthetic piston head with a very thin smear of silicon oil upon reassembly, such as the product from the Bisley range, it comes in a small dropper bottle.
Use a medium to high temperature moly based grease for the metal to metal surfaces or one of the white long lasting greases supplied by Anschutz or FWB as stated by 18 wheeler. I prefer the former as the cocking lever components on the Walther are heavier & don't contain synthetic components unlike the FWB & Anschutz ssp's.
IF IT'S NOT BROKE.........DON'T FIX IT!