You need to increase the spring rate and the damping to match. Heavier oil will just increase the damping which without the harder springs will slow the fork down, in particular the rebound. So what will happen is you will go off a small drop, the fork will compress, you will go over the bars, and then the fork will rebound (slowly). When I was young and naive I was told to do this with a set of hydracoil Judys, instead of spending lots of money on expensive springs. I put 15w in (I think the standard was 5w?) and to be honest it wasn't too bad to start with, but that because it was summer and I was riding on hot sunny days and the forks were painted black. However when the weather cooled it all started to go pete tong. Eventually after going over the bars 3 times on one ride I changed the forks to a set of Duke XCs. I later fitted the Judys to my SS and changed the oil to 7.5w and they were fine again; I'm lighter now so the springs are fine with the pre-loads wound right up. I dont know much about Manitous but if the standard oil is around the 5 to 10w then putting 30w in sounds like a bad idea to me, even more so at this time of year.