I had a 3GM30F in my last boat. Having owned it for over 20 years, it was a very reliable engine that needed very little attention. With clean fuel, air, and oil it was pretty much bulletproof. I did have to change the exhaust elbow several times. They tend to get clogged with soot and corrosion. They now make a stainless elbow. I put that one most recently. A great innovation and an attractive addition.
Jake