I was told the same thing by multiple mercruiser mechanics

Hard use gets these engines more than anything. Makes sense. I have small boat for such an engine and do 95% of my hours under 3000rpm. The last boat I had I did the same and the 1979 Mercruiser 228 lasted over 3000 hours unopened.

I agree about use. Mine have about 410 hours on them and they still hit the same top end speeds and r.p.m. like they were when new. I run them pretty conservatively though. Alot of 3000 or less r.p.m. cruising, never run the engines hard unless there fully warmed up, boat dosent get any air time, 25-30 hour oil changes, etc.

I agree also. Mine have about 400 hours as well. They' re babied and run great. I did replace the exhaust manifolds with Stainless Marine, but that was preventive. They do use a little oil at the normal level to be expected. Nice part, is just turn the keys and go.

Same story here with the 502's. I replaced the exhaust with CMI's and replaced the flame arrestors with Vortec intake and K&N filters. Right around 390 hours of trouble free operation. Have run some poker runs at WOT, but always provide a cool down before shutting them down. Mostly cruise around 3500 RPM's. One engine uses about a quart of oil every 10 hours, the other one every 15 hours. This takes into account some WOT time, if only cruising they don't use as much. Let them warm up before you put the hammer down.