This is not uncommon to get carbon build up even with the Mobil 254 HTS oil. I always recommend cleaning the tube at the 600 hour inspection. Some aircraft seem to do it more than others.
Do you find yourself spending alot of time at ground idle? Do you perform a relatively high amount of start-shutdowns per flight hour? If you ever get the chance, look at the vent pipe during a start sequence and you can often see a fair amount of oil venting before the engine gets up to speed and the pressures all get to where they need to be.
It is possible that you have a breather seal leaking which is making the situation worse but it is not very common at all in my experience.
Also be careful with the small mounting tab on the vent tube that attaches it to the exhaust duct. They have a tendancy to crack and break (at least they used to ). Many folks just cut off the bracket and secure the tube with a large adell clamp. You need to make sure the tube is properly positioned into the exhaust stream to get the proper effect. We also had one that cracked at that point and would make a huge mess all the time. We changed breather seals and no luck. Cleaned out the tube and no luck there either. Once we finally pulled the tube off we found the crack on the back (inboard) side which was causing the trouble. Tube changed and all was well again.
Hope this helps!
Max
Last edited by maxtork; 24th Nov 2009 at 13:06.
Reason: typo