I've been using Mobil T-1 10w-40 full synth for +50k miles and no issues. I ride anywhere between 5 and 125 degrees and change oil between 6 and 7k miles. As I understand its the only oil used in my bike after the break in change, but I'll never know if it is better or worse or if any abnormal engine wear is occurring until its too late... I really don't foresee and issue and I'm not cracking the case to find out without good reason. On my previous bike I used Castrol GTX as somebody mentioned earlier but it had a dry clutch! Castrol GTX has friction modifiers and that is by definition energy conserving; Castrol only recommends its "Power 1 Racing 4T 10W-40" for Goldwings, I would assume that is because we have wet clutches. Frankly the other rust preventative oils make me scratch my head as oil itself should be rust prevention by itself, but my wing isn't parked long enough for dust much less rust!
Tom, in Mountain Home, Idaho
2002 GL1800 (Illusion Red) Non-ABS, 128k miles
Retired Air Force
"Audentes Fortuna Juvat"