My experience with full synthetic oil, Mobil 1 motorcycle oil, I put it in the R65 in November '99, in March 2000 the rear main seal started leaking to where I had to do something about it. I replaced the rear main seal, and 4 weeks later the rear main seal started leaking the same amount as the old main seal. I drained the Mobil 1 out of the bike and replaced it with Spectro Golden 20-50w, ( the oil that I had been using for the previous 15 years ) and the leak went away with the next ride. I personally won't use a full synthetic in the engine again. I'm not so sure about using a full synthetic in the drive train either, as these seals are made from the same material as the rear main seal, Viton. I use BMW gear oil and change it at least once a year. When I had my transmission overhauled about 2 months ago, other than rust damage to the bearings from water contamination from a bad speedometer boot, no problems at all at 80,000 miles. I'd like to hear about other owners experiences with synthetic oil in the drive train, as with the unrelenting heat here in Phoenix, I'd like to have that little extra protection for peace of mind!