Back on topic,
I'd let it pass for now (fighting against the early change).
You know, changing the oil early is only going to hurt the pocketbook, nothing else, maybe the environment the slightest bit.
Therefore, let it happen.
Do you change the oil or them?
If it's you, down the road, they will forget about it, and you can change it when you want to. If it's them, let them do what they want I suppose. It would be worse if they wanted you to go 20k on oil. At least they want it done more often, which doesn't hurt your car.
Now, in terms of the viscosity they want to use, that is more annoying. I don't know what to tell you about that. If you do it without them around, I suppose use what you want, unless they'll find out.
This is just silliness in a way though isn't it? ha. Good luck!!!
Side note, a few years ago I would have been like the father or buddy mechanic. I would have wanted the oil done at short intervals even if synthetic. You could not have convinced me otherwise. Therefore, you can't blame them completely for wanting to do the "right," thing in their head for the car.