If I am not mistaken, 10-22 is 11% castor and 11% synthetic, correct me if I am wrong please.
there are 128 ounces of fluid in a gallon,
in 128 oz, there is 22 percent oil that equates to about 28 oz of oil,128 * .22=~28OZ
by adding 8 oz of oil, you will now have 36 oz of oil.
so,
128 = 8 =136oz total mix, 36 oz oil / 136 oz fuel = 26.5 % total oil, see some difference
so to get closer,, lets try 10 oz added oil
128 +10 =138  now there is 38 oz oil,, so 38/138 = 27.5% total oil

128+11 = 139  now there is 39 oz total oil,, so 39/139=28.05 % total oil

Thats how i get there,,

OK Everybody:

  Ole "Billy G" is gonna make this easy for you. Try this:


         http://www.nitrorc.com/default2.asp?Introduction=http://nitrorc.com/fuelws

   Read it if youwish but if you scroll to the bottom there are 2 boxes. One say oil the other nitro & oil--Click on what you want,fill in the blanks enter it and it does all the figuring for you. Simple. Go ahead and play it will give you the right answers every time, no guesswork.

  "Billy G"   

Randy;

  I guess that means that you think this site is useless for mixing your own fuel? I'm done I won't post helpful info ever again.

  Thank-you;

           Bill Gruby 

  If what you say is true about some manufacturers mixing by weight, then all the replies, yours included are guesswork because you really don't know what you have to start with? Yet you chose to quote mine, why is that?

Maybe some one can help me out with a small problem, I bought a gallon of Powermaster 10 -22 fuel.  Ineed 10 -28 how can I get to 28% castor with out lowering nitro to much? Its for a fox 35    Thanks John

John,
It depends on which Powermaster 10-22 fuel you started with. There are two: the "regular" 10-22 which is 11% Castor and 11% Synthetic (which you will never get to 28% all-castor!); and the GMA All-Castor 10-22, which is 22% Castor Oil (This one can be made into 28% All-Castor - Just use the numbers provided by others and add around 10 ounces of Castor Oil). The resulting % of nitro will be reduced from 10% (however it's figured by weight/volume) to around 9.3%, which isn't a noticeable change for anyone!

If I'm guessing correctly, you want the 28% for an old Fox/Johnson/K&B using an iron piston in a steel cylinder, and 28% all-Castor is proper. If the fuel you have is the "regular" 10-22 with 11% Synthetic Oil, save that for the modern stuff, like Brodak or OS LA/FP engines, and go buy a gallon of Powermaster GMA All-Castor 10-29, which is a fine blend for the old engines. This'll save you time and effort, and really won't cost any more in the long run; have you priced AA de-Varnished Castor Oil recently?

Thanks for all the help ,I have all the info I need.             John