Standard rule of thumb to stay safe and not void your warranty is to stick to the warranty term. Not sure how that has been outlined for your G80's year model but I suggest you look into the details. Its those very words they'll base a claim under.
I would confirm with your dealer, but I would be very surprised if moving to premium grade oil was seen as an infringement on your warranty. Mobil 1 has a semi synthetic 5W-30 that would make for an excellent choice in the G80.