I've got a 21' boat with a Jasper Marine 350 in it. This is my first inboard/outboard boat. I've heard that straight 40W is the way to go. What about 15W40?

Dude. I run sythetic, But that question is like asking
"what kind of beer do you drink?, and which one is better?"

You'll get 1,000,000 different answer. and they are all good answer.

Run a good quality oil in the viscosity recommended by the engine manufacturer and youll be fine.

whaty does merc recommend for a 330hp 454????

Hey Puder,
They recommend you rebuild the ***** into a fire breathing 525 sc !!!
New crank, rods, pistons, cam, bearings, blower, heads, and exhaust!!!!!

Just kidding my brother!
I was going to run Mobil 1 synthetic
Did wonders for my trans am so I figured it would improve this tank of an engine as well
Might take 2-3 changes to completely wipe out all the natural oil and get lifters all pumped up wtih synthetic but hey, have to start somewhere

I run 20-50 in my engines
sometimes straight 50w
All synthetic.....RedLine Engines/trans/outdrives

OH NOOOOO!!!! Not another oil debate!

I'd stay away from the straight weight oil. Mercruiser recommends 15W40 for the small block (at least they did in 2000). I like to use Shell Rotella T. It's rated for heavy trucks, and exceeds all manufacturers' requirements. $5.99/gallon at Pep Boys!

see what I mean

Run a quality oil and change it OFTEN. I usually run a 20-50 and am not totally sold on sythentics. So many choices.