I think a 2-ounce oiler would be good for 100 ounces (3 quarts, 4 ounces) to mix with.
Do you know how thiese oilers work? What controls the oil drip? Am I right to assume that this screws into a non-pressurized oil line?
Why is there an open area between the oiler and the threaded part of the fitting? Is that so you can watch how fast the oil drips into the oil line?
I would have a threaded fitting brazed/welded to the tank, near the gas cap. Then I would fill the oiler the day before filling gas and install it onto the tank. Upon reaching home after filling the tank, I'd remove the oiler and plug the tank's fitting.
My HT tank sits on the rear rack, while my engine is frame-mounted. On my next Project 29er, the engines will be rack-mounted, and the HT tank will bolt onto the bike's top tube. If the brass drip oiler could be protected from falls, I'd leave it permanently mounted.
From what the pictures show, it is a beautiful piece of old-fashioned engine component.