to honest i would like steptoe to elaborate if possible please,as i have been using 75w/90 ,gl5 spec for years ,am i using the wrong gear ,and thats in my r1100rt and my 2010 twin cam gs ,in both the gear box and the final drive ,its not expensive so when it gets an oil and filter change i also change the final drive and the box as well.
From what i can see , and i dont have your exact model details it says an 0413 series 259 R1100RT should be a Hypoid gear oil SAE 90 API GL5
If your not sure phone your local dealer and ask the service dept, they can look your bike up in minutes and see whats specified