I look at specific gravity to get a general idea if the castor oil will separate from the gasoline and or at what fuel temperature separation may occur. Some of the fuels like 100LL av-gas may separate at 80 F unless some type of blending agent is added to the base fuel.
The "jar" test is the best way to test the fuel for separation.
I think 100LL has a specific gravity of around .68 to .70. Fuel with low aromatic content typically has more problems mixing with castor oil. Fuels with a specific gravity around .74 or higher, most often have higher aromatic content and castor oil will usually stay mixed in the fuel to around 50 F.
Unleaded pump gas has a high aromatic content and can be used as a blending agent in leaded race fuel or 100LL av-gas with out appreciably affecting the octane rating. For most of my customers, I advise them to mix one gallon of pump premium to 4 gallons of race or av-gas.
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