I think knowing your own bike is important here, because for every person who has a GS that hardly even sips oil, you'll get someone like me who owns one that guzzles the stuff. Mine is upto 11k miles now and has had several litres of oil between services. Thankfully, much in line with what my dealer told me, that does seem to be settling down now.
Personally, even with a guzzler, I'd not bother taking oil for less than 750 miles, just a check that there was plenty before I set off.
Mine is going on a trip in a couple of weeks and I'll definitely be taking a litre along. Just one less thing for me to worry about I guess.
When I bought the bike lots of people told me that boxers use a lot of oil so I always carry a litre round with me just in case, my question is: Do I need to, or am I just being a tosser?
My experience with this is very much in line with what is described by others here. Bike (05 model) used quite a bit when new and then slowly got better.
Anyway, without having any intention of starting a new "what oil to use" discussion and getting shot for that , I'd like to mention that when changing from dino to syn oil at about 15000km the oil consumption seemed to drop quite a bit to nearly zero.
I do not know exactly why but seem to remember to have read that syn contains less of light weight stuff that tends to evaporate over time.
On longer trips I carry a little bottle (300ml) of the oil I use. Even it sounds logical that you can buy oil "everywhere", you hardly get it in the quantity you need for topping up, do you?
What most likely happens is this:
You are turning in at the first petrol station and don't find what you need. You finally buy a liter of an oil that would not buy if you would not have to for a price that you hardly would find acceptable at home. You then fill in some 300-400ml....and then you are carrying the rest don't you?