higher rpm increases crankcase pressure if i remember right so i would check the things listed above and if no holes or leaks are found i would upgrade the crankcase evac system to relieve the pressure better if you plan to spin 6000 again
xs2 .... when i first installed my MP valve covers, i started with just a breather and blew oil out on my dipstick tube and around the breather. so then i drilled out the opposite side and put in a PCV going to the carb and never looked back, problem fixed