For a 1990 model 90 hp Evinrude outboard motor, the recommended fuel mix ratio is 50:1. This means that for every 50 parts of gasoline, you should add 1 part of Evinrude XD100 outboard oil or another NMMA TC-W3 certified 2-stroke oil.
Note: Always refer to the owner's manual for your specific outboard motor for the most accurate and up-to-date information on fuel mixing ratios.