I recently looked up the viscosity of gasoline (15.6 @ 60*F) and then figured out the viscosity of different engine oils at the same temperature. Then I averaged the viscosity for two fuel/oil ratios, 30:1 and 50:1 and found significant differences. Castor 927 with gas had a 33% viscosity increase going from 50:1 to 30:1 and Motorex had a 12% increase. I had no idea there was such variation.
Have any of you noticed a need for different jetting when you switched brands of engine oil but kept the same fuel/oil ratio?