JP,
Try this stuff called "slime". No kidding. It goes by a variety of names but it is used by electricians to pull cables through rigid conduit. You can get it at Home Depot or Lowe's in the electrical department. Does not harm anything, and the excess will easily be wiped off.
One manufacturer to look for is GB, and their version is called "Wire Aid". Good Luck!
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America