ORIGINAL: TedMo
No authority on this but how will you know when you have added what amount needed? also 20% nitro is a lot for most planes. Perhaps an authority will give you better answers good luck.
Some basic math will allow him to figure out how much oil to add. Adding oil will also
slightly reduce the nitro percentage compared to the total volume. Some engines actually LIKE the extra nitro and since he didn't state what engine it was destined for we will need to let him decide that issue.