marzocchi forks have 7,5 wt in there when theyre new,so yeah,should be thicker.
you can adjust the rebound internally with a hex key though,i´d try that first before putting higher viscosity oil into it,those forks have a ssv valve system which means your compression and rebound use the same oil,leading to an always slow rebound with too thick oil,fork would stay compressed when going over rough terrain.
for my ols marzocchi fork a selfmixed 8,5 wt was best,anything thinner topped out,anything thicker screwed up the adjustability.
main problem should be the spring if its topping out too easy,maybe you need a harder one