My two stroke motorcycle riding experience has shown me that a rich fuel to air ratio will foul plugs a lot faster than a lot of oil in the fuel. In the olden days before glow plugs were invented, a standard fuel for small model engines was one part 70 weight oil to two parts white gasoline. If your air fuel mixture is right, I think you will be able to run 10% oil without problems and your Saito should be happy.
You might also consider using E-85 and Klotz instead of gasoline.