Yeah, I'm with Pete on that, Pisano.
When I got my GRiSO I too followed the instructions in the workshop manual & the breather cap just crumbled away when I got the spanner on it; fortunately it left a piece sticking out so I could get the mole-grips on it to unscrew what remained.
I was surprised how pricey the replacement was seeings as it's just a breather plug!
I'm many miles & oil changes down the line since then on my GRiSO & have always left that breather well alone.
My advice; don't touch it!