There will generally be mayo in the breather tank as it seperates the oil/water thus creating the mayo, nothing wrong in that. I have a -8 welded to the sump where i return to but i'm not sure it's really needed on an rst having asked the same question on here and finding the other big power boys don't seem to bother with it. I'd do it in -10 next time i think, they do it on cossies/evos etc hence i was advised to do it on mine. In all honesty there shouldn't be much going back in unless your engine's breathing really heavy so a catch tank should be fine.