Yes, I use Oxi for everyday cleaning, I use PBW for stubborn stuff. Also, save yourself some headache. Rebuild the whole thing. Take off the dip tube and let it drop into a full kegs worth of PBW solution. Heat the water first. 175 degree pbw solution can take care of even the most stubborn stains.
I would probably
A) Preheat 4 gallons of h20 to 175. Put it into the keg (it'll be HOT) wear gloves and long sleeves when handling it!
B) Add a full strength solution of PBW (usually one scoop for this much h20).
C) shake it like hell (It's hot! don't burn yourself!)
D) Pressure the keg with CO2 and run all 4 gallons through the diptube/liquid out post into your stainless kettle.
E) Take off the dip tube, both posts, and gas in tube. Put the small parts in a smaller container of PBW solution, drop the diptube into the keg so it completely submerges. Scrub the small parts.
D) rebuild the keg with fresh Orings.
E) pressure it slightly and leave the PBW solution in the keg. Turn it upside down for an hour or and hour and a half.
F) Turn it right side up for an hour or an hour and a half.
G) Dump the solution (it's always best to clean all your empty kegs at one time for this reason IMHO, less waste).
H) Rinse with HOT H20 thoroughly. Fill with 2-3 gallons of uber hot h20 after it's completely rinsed.
I) pressurize it with Co2, and run the entire 2-3 gallons of clean hot water through the dip tube and liquid out post.
J) Fill it with Starsan solution
K) pressurize it and run the starsan completely through the liquid out tube. Shake it around too to get the starsan on the entire inside
L)Your done. Leave the c02 in it store it pressurized and sanitized.
I know that's a lot of steps, but I can virtually guarantee you'll have a clean keg, with no gas or liquid leaks, and no dispensing problems. It feels great when you line up 6 kegs and just run the liquid through them one to the next and when you're done you feel like you can brew 6 batches of beer and be ready to roll!
Hope this helps.