To prevent overfilling which may damage your engine I suggest you only top using 100/200ml slugs. Then leave the bike to stand before checking the level again.
As you are only likely to need about 200ml of oil to raise the level from the bottom of the sight glass to just above the mid point, it does not really matter which brand of oil you use.
200ml of 15w/40 oil added to a sump full of 10w/40 is not going to alter the overal viscoscity significantly.
Your owner's manual will tell you the spec of oil to use.
For example API SG
Therefore, if you have a can of oil in your garage that meets this spec then use it for top up purposes. Castrol GTX will do.
For an oil to get an API classification it must mix with all other oils in the same and lower classifications. (This only applies to oils for petrol engines, diesel engine oils do not have to meet earlier specs).
This stuff about not mixing brands, mineral or synthetic is complete bollox.