You  do not want 50% oil!  Actually adding oil does not lower the nitro content, which seems to violate the laws of physics.  Look at it this way.  Your engine is adjusted to run well on a certain amount of alcohol and nitro finding their way into the cylinder.  The oil is just along for the ride.  If you add more oil, you have to open the needle so you can get the same amount of alcohol and nitro into the cylinder, along with more oil. 

I would suggest you buy some fuel with 10% nitro, whatever kind of oil, then add castor to get the total oil up to 25 % and go fly.