Don’t neglect the transmission for your inboard motor, as you don’t want to be left stranded if it fails.
The below tips will assist you in taking better care of your marine transmission.
Look for signs of leaks running down the case. Are any seals sweating? If so, have a professional look at it. Also check to see if there is any sign of water dripping or leaking onto the transmission housing itself, which could cause corrosion.
Deploy a catch pan and absorbent cloths, remove the plug, and remove the dipstick to provide a vent. If your transmission does not have a drain plug, use a pump to drain the fluid via the dipstick tube.
If your transmission requires automatic transmission fluid, it should appear as a translucent, reddish-pink color. If it requires standard oil, it should be a maple-syrup color with no milkiness (water) or particulate grit. Oil should not smell or look burnt. If you find any of these things, call a professional. Change fluid annually. Always use the manufacturer’s recommended oil type and weight.
For the full article, see: https://www.boatingmag.com/story/how-to/marine-transmission-care-tips/