Changing the lower unit gear oil in a 2014 f40 Yamaha.

Valvoline 80W90 gear oil: https://amzn.to/2NILRF0

I recommend changing the oil at least once a year, but if you use your boat a lot like I do, you're better off changing it more often. I change mine every 4 months or so. The peace of mind that comes with knowing everything is working properly is well worth the 15 minutes it takes to change the oil.