A unique website dedicated to fishing information from Florida's Northern Big Bend. This includes the area from the Econfina River west to the Apalachicola River
Here is what a mechanic at a Yamaha dealer stated. Prior to 1985 use 50:1 after use 100:1 unless you are working your motor extremely hard all the time than 50:1 for any year.
Yamaha's web site has owners manuals to 98' they say 25:1 during break in then 100:1. Personally I have an older Evenrude 6 horse that says 100:1 and I use 50:1 just to keeps things lubed good.