Finish Line Cross Country Synthetic oil - it's for bicycle chains. Comes in a black bottle with a green top. About £5 for as much oil as you'll need for a lifetime!
RC bearings need a synthetic oil as they have very high point loads and, in the motor, not a little heat. Synthetic oil does not gum up, and has good shear strength. Much the best oil for our application, and very easy to get from any decent bike shop. HTH
