Well, it's really bad news if 5th gear runs without oil and 5th is the one that will suffer if you are even a bit low. If you really want to cheap out (and gamble that you will never need to top off) better to add a dash of something. Even motor oil will be better than air for 5th gear.
I would never gamble on not having 100ml of oil in the transmission personally.
Thats 5% less than recommend, Im not mechanic but always been lead to believe oil levels in transmission is crucial!
This is what I did and ran it by the Master BROKEVW and he said that should work. Since mine is "new" I wanted to make sure it got what was recommended best. I asked brokevw what he was running and he wouldn't say. He didn't want to sway people one way or the other. I asked and ended up running one quart of syncromesh and one quart full synthetic 75w90 then the rest filled with 80w90. That way the syncromesh is thickened just a bit with the 80w but it isn't too thick. The syncromesh is noticably thinner than the 75w90 so that is why I used the .1qt of 80w90 as I figured it would end up just right. I think when done with this round after a few thousand miles I will go with just a full syth of high quality like amsoil or redline.
From the stories I have read on vortex about second gear grind the best oil to use is not synthetic , but dyno 80w-90 gl4 oil. good luck finding it lol.