What is the fuel oil mix ratio for a 1979 mercury 90 hp?

If you want it to last, use a name brand oil mixed (really good) to a ratio of 32:1. That's 1 quart oil to 8 gallons of gas.

If you use Mercury brand oil and mix it really, really, really good, you can go 50:1. That's 1 quart oil to 50 quarts of gasoline.