Too me, that is a missing bolt hole. oil works it way from the the Oil sump,(under the Oil fill cap) then the Oil pump pushes the oil thought the filter, to the Head, and stator side, With all that said, Would gas mess up first gear? Sure its possible the oil we recommend is JASC MA, if the fuel can water that down, and you can possible lose the neccessary stuff for Clutch lubrcation.
Next steps?
1. Smell the oil under the oil fill cap. does it smell like fuel? Drain it, replace filter and refill with MA coded oil. (Shell Rotella-T is highly recommended by me)
2. No fuel smell? What type of oil did you put in their last time? was it MA? Drain it, replace filter and refill with MA coded oil.
3. Clutch adjustment could be out. See thread
http://www.xs400.com/forum/showthread.php?t=10687&page=3
# 33, 50, and 60 something. Have pics, and or manual printouts.
4. if 1 or 2 didn't solve your problem? did you bend you shift armature? could you have been shifting with out the wheels moving? I am thinking of 3 things attached to the gearing stuff, that affects gearing shifting, not including damage to the 1st gear sprocket.
I have even had the clutch lining get out and screwup my clutching, and getting in and out of gears.
Good luck!
tobie