Yamaha specify that the FJR final drive oil MUST be fully synthetic. There are alternatives to the Yamaha product out there, usually under the "racing" banner. Just as long as they are fully synthetic. Standard 80/90 gear oils are not! This comes about because the final drive on the FJR is pretty much the same as other lower powered models like my old XJ900S (are they called Secas in USA?) which will run on standard 80/90 and only need changing every 16,000. The FJR needs a little help to cope hence the synthetic oil and the 6,000 mile change.