There won't be any adverse chemical reaction if that's what worries you. Even synthetic and dino oils can be mixed without any ill effects...but my personal opinion is that it's best to go with one or the other.
Some mfg's suggest you can if on the road, but should change the oil when you get back because not all additive packages are compatable with each other.
When I go on long bike trips (just got back from 3500 miles into Canada and back) I usually bring along a quart just in case. I wouldn't have any reservations about mixing oil for a short time. When I was younger, and oil cost around 30 cents/qt. I put in whatever was on sale. Now I use Amsoil synthetic exclusively because it is one of the best oils and I get it at dealer cost.
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