Your 4.3 is an automotive-based Chevy without any smog controls like a catalytic converter. If you want the best protection, use diesel oil instead of gasoline powered auto or marine oil. Non-diesel oils are formulated with far less Zinc Dialkyl Dithiophosphates or ZDDP due to the fact that ZDDP damages gasoline engine catalytic converters if it makes it into the combustion process. Since you don't have a catalytic converter, you can get the advantage of a lot of ZDDP (which is fantastic for preventing engine wear), so you might as well do so. The best part of the whole deal is that diesel oil is typically a lot cheaper than gasoline powered auto oil.
As far as viscosity goes, the number preceding the W is irrelevant as along as it is lower than or equal to what the manufacturer recommends. 5W-40 protects at high temperatures just as well as 25W-40. The 40 is the key parameter.
Unless you are putting more than 200 hours on your oil between changes, or you are using the boat in weather well below freezing, there's no advantage to synthetic or synthetic blend oil.
I'm a huge fan of Shell Rotella T4 15W-40 in applications like yours, I buy it in 5 gallon buckets and it's great stuff for very little money. If you really want synthetic, T6 5W-40 is also available.
For those that have marine engines with catalytic converters, you really need to use oil that meets or exceeds the recommendations of the engine builder. Don't use diesel oil.
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