Yes only a fifth of a gallon is not going to be a big issue as long as you have no lean runs i.e. setting the needle too lean. What ever fuel you purchase next I would suggest a minimum a total of 18% oil. That can include a mix of synthetic oil and castor oil. It is my belief that even a small amount of castor in the oil mix is fine and even low as 3% of the oil mix is castor works well.

Yes that engine is the one I had in mind and my memory is vague but I believe there is a way to defeat the limiter, a picture of your needle valve would help here. If you intend to operate the engine with the limiter then you need to search out the OS operating instructions and adhere only to them. Without the limiter then we can treat the engine as a conventional one an there will be a plethora help on the forums in that situation. I think your reluctance concerning the needle valve operation is perhaps the lack of a proper extension. Which normally is quite easy to provide. Virtually every OS that I have ever seen since my first OS in the sixtys has provided a method for a needle extensions of one form or another.

John