Hi mate
Thicker oil will only work if you have the updated late 09/2010 open bath rebound cart installed. Way to check is to simply jump up and down on the fork, and if you here a squelching sound then you have the open bath damper installed, and yes, changing to a thicker oil will affect the rebound (make slower with heavier oil) Marz forks often work better with a thicker oil
however if you have the older style nitrogen charged R cart (the most hideous and pathetic damper Marz has ever produced IMO) then no, changing the oil wont work because the damper is fully sealed, and will more than likely fail and lockup on you