Gipper, you have an electric shift gearcase, and that takes the lowest viscosity stuff as suggested, similar to ATF. OMC sold it under "premium blend" in '75 when they said the low vis Premium Blend had to be used in the '62 - '72 electric gear cases. If you choose to use ATF, use Dexron -- think your Mystic label reads "Certain other electric shift gear case units require DEXRON-III type fluid. Consult Owner's Manual." You have one of those gear cases.
Type F transmission fluid is hard on seals, I think. And I guess I would want to check the Starbrite lube for viscosity before using -- think the Starbrite recommendation re. electric gear cases is suspect, just given the confusions surrounding Type C use (somewhat murky OMC instructions). Premium Blend is probably still around, and that would be the best bet -- but guess that's the expensive one you wanted to avoid. I would spring for it, though -- used it for about 10 years in a '70 85, and had no gearcase issues (saltwater use, most of the time).