A friend bought a used 07 RK and complain about the 5th gear noise. We went too Ray Price in raleigh,nc (HD Dealer) to pick to up the oil for all the 3 holes. I ask the parts guy for Amsoil 75/90 gear oil and he said he didnt carry that weight. He did carry Wynns synthetic 75/140 gear oil and wynns friction modifier for the 07 with 6 gears. which would stop the 5th gear noise. He told us to pour out 4 1/4 out of the wynns gear oil and put 4 1/4 of the wynns friction modifier back in the gear oil bottle and mix them up. he said that would fix the noise..well we did just what he said and sure enough the noise is gone. The service dept has been using the wynns for 2 years now for the 07 with 6 gears.
I google wynns and nothing came up for motorcycles or Harleys. Does something work any better than wynns? or do you think this could harm the transmission?