You definitely want to have proof of your maintenance intervals for any possible warranty concerns especially if you do it yourself. I do my own maintenance and I do a short video on my iPad of each oil change by starting the video showing the mileage on the bike and then I pan down to the oil that is draining out and show the oil filter has been removed. I then include a shot of the oil I am putting in and the Honda oil filter. I highly suggest using the factory Honda oil filter until the warranty has expired so Mother Honda doesn't try to find a reason to deny warranty. My videos are date stamped so if I ever need warranty I would think the video and receipts of correct recommended oil/filter would be sufficient proof the maintenance was done properly. 4,000 miles is about right - synthetic maybe a little longer but you want to make sure you stick to the 4,000 mile recommended interval for warranty.
I wouldn't speculate having any problems to begin with as the GL1800 engine is probably the most reliable and bullet proof motor you can get in a motorcycle!
I also video certain "things" my wife and I do not because I am weird, but only because I need proof sometimes that is has been a while. The date and time stamp always prove my case.