1980 100 horse evinrude, new to boating and need to know how to change lower gear oil, where is the plug, what kind of oil, and need to know the mix of gas and oil? Thank you very much for any help!
Here is a picture of where your oil plugs are.
First you drain out of the lower plug #53
Remove vent plug #45
Fill with new oil from the lower plug till oil runs out of #45 plug.
Most gear oil containers have a spout that you cut off with a knife and fit that spout into the lower plug hole, and squeeze the bottle till oil runs out the top plug.