A good dose of preventative maintenance before you leave will be more valuable than anything you carry.
I'd change the plug at home and not carry one at least for the first 15000 miles. I'd also avoid fix a flat - that crap rusts rims (when they are steel) and messes them up for the future. If you want something for flats use Slime (temporarily to get you home then buy a new tube) or (whatsitcalled) Rollon? a special MC flat preventer that you insert in the tube before the flat happens. Forget the quart of oil unless you are burning it like crazy. Maybe a jigger full would be nice for a month long trip. I'd take the bike tool kit, maybe an extra crescent wrench, and enough navigation equipment to let you walk out or enough water and snack to await help. Somethings you just can't plan for - so lean back and enjoy the bike without bogging it down with stuff that you will never use on the trail. JMHO. Tom