Doesn't matter too much, I usually moisten it. 3 reasons; 1 - usually my reason - I use grease or oil just to help hold it in place a little better when assembling so you don't have a section slide out or punch unintended holes when shoving the case bolts through. 2) it may help even compression of the gasket material (not likely significant) and 3) it DOES help remove it again later. like if you fuck up and forget some tiny piece of crap and have to split the case again next week, it's still reusable if you oiled it. and if somebody else in 10 years wants to rebuild it again (maybe you?) it's easier to clean off the old gasket. so yea, oil it. why not!