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It has a tendency to find spots that don't currently leak but are a little weak, and make it a full fledged leak, and after its leaked a little synthetic through it, conventional oil will seldom stop the leak again, rather it'll just find the now bigger "hole" and leak through it as well...and if it burns oil now, it'll burn more on synthetic for the same basic reason.

a synthetic will undeniably provide superior protection from extreme temperatures, a fact i consider when riding an air cooled bike in the summer. i ran synthetic from day 1 in my fj1200, and when i traded it w/ 58k the mechanic was impressed with how tight it still was.

a cb900f i had expired one hot summer day when run on regular oil.

not conclusive, but a fact. i run synthetic in my water cooled bikes now too, although i think its overboard in an automobile unless one is into extended drain intervals.

Synthetic is fine oil to use, but if the bike already burns/leaks oil, its a bad idea to switch to synthetic, since its notorious for making leaks and burning worse. I agree it does protect better, but if it goes from a little puff of smoke here and there to looking like a fog machine inside of a few months, you're really not helping it any.

If I ever find a dirt cheap (read: free) kz550 motor that doesn't have anything severely wrong with it, I'm going to consider a fairly complete rebuild and start it on synthetic from day one..but until somone shows up on my doorstep, or atleast within 60 miles of me with said motor, that doesn't seem likely.