In regards to the original question.
#1 you have a stone age engine where the transmission is separate and not shearing oil molecules. The clutch is dry and not putting crud in the oil. That boxer isn't screaming at 10K rpm, even at 70 or 80mph, its loafing. It has an oil cooler and those jugs are right out their in the breeze. Your oil will be good for a long time.
#2 as has already been said, you are doing long distance riding which is the easiest scenario there is for engine load.
#3 the oil change interval is ALWAYS a CYA number where the manufacturer expects you to exceed it.
#4 on the GS the synthetic versus dino stuff is pretty much right up there with voodoo. Do you know what is in the bike now? With a GS it is certain you will have to add oil. My superstition tells me to use similar oil to top it off rather than mix different stuff. Probably doesn't matter but it makes me feel better.
I know we love our bikes but I think your concern is without merit.