If I was going to change my foot gear oil in a Mercruiser Alpha drive
after every use (about every 4 hours). I do this due to a small leak
in one of the seals that allows about a half cup of water in the same
time period to enter the foot (just until I can get it repaired),
would it really do any harm to use the cheaper automotive 80W90 gear
oil vs. the marine grade stuff?

I have received one answer regarding this but would like more
opinions.

Thanks to all....you all have been very helpful.....per usual.

Dave