Jim:

You are ignoring the increased total volume.

One gallon with 17% oil - 21.76 ounces of oil, and 106.24 ounces fuel component. The additiion of 6.4 ounces of oil leaves us with 28.16 ounces of oil in a total volume of 134.4 ounces. run the numbers yourself - 28.16 ounces of oil is 20.95% of the 134.4 total.

On the other hand, adding 8.2 ounces to the 21.76 then gives us 29.96 ounces oil in a volume of 136.2 ounces, 21.997% oil.

Forgetting that the total volume goes up as you add oil is where almost everyone gets messed up.

Bill.