We have had a similar issue with our 2000 ISC, except ours only throws oil out the tube when we are using the Pac-Brake. Apparently this was a problem that Cummins was aware of for a few years. After talking with Scott at Cummins USA, he said he was aware of the problem and recommended using a crankcase ventilation system, the Cummins part numbers were CV-50111 or CV-50202 depending on the amount of oil blowing back out the tube.
May not be exactly the same problem as yours, but maybe the same fix?
Update on the breather tube issue!! After my quick fix with rescue tape (if you don't know what that is, here's the link to their site http://www.rescuetape.com ) We made it to Wyoming with no oil blowing out at all! We did have one issue coming into Lincoln, NE which caused a bit of a freak out for us. We started loosing power going up hills........and those hills aren't huge! So we pulled into a freight liner repair place and offloaded the dogs, some of our stuff, etc. anticipating the worst and settled into a hotel. I thought turbo for sure and maybe a trip cut short, but it turned out to be a bad fuel filter. Which confounded me, since we just had a new one put in and we ALWAYS do our regular maintenance on the engine. Well, it turns out to our surprise that there are TWO fuel filters, one further back in the engine and the mechanic explained that service folks are notorious for not changing that one and when he pulled the rusty old filter off if was the ORIGINAL filter! Are you kidding me??? In 8 years not one mechanic who serviced our coach bothered to change that filter???!!!! Needless to say, I am a lot smarter and our normal service dealer back home is going to get an earful!! Good news is, our coach runs like new now and all is good!! But our service place in Tampa is going to have a 5ft. tall bundle of fury to deal with when we get home!!
Another update on the weird oil blowing issue. After several months, up and down mountains, long days and short days of driving, hot days and cold days, the "rescue tape" fix is still working. Not one drop of oil blowing out......and since we now know there are two fuel filters and they are BOTH clean, the coach is running great.