This is my first post and I have a few questions that you guys can help me with. I am the process of having a new 5.7 Vortec Marine 320 hp carb version installed in my boat. Bolted to a Cobra outdrive. A engine oil cooler has been installed as well.

1. The shop that is doing the install states I should not use synthetic oil during the break in period. As the rings may not seat. But I know the new Corvettes come with synthetic from the factory. Is this a problem?

2. Oil recommendation from the Marine engine company is.
Ambient temp above 50F - API SAE 15-40 SM
Ambient temp below 50F - API SAE 5-30 SM

3. I ordered a case of Amsoil-4-Stroke Marine Synthetic [WCF] SAE 10W-40. I notice it is not SM rated but on the Amsoil site it shows it is ok to use in an I/O. Is this the correct oil to use?

Thanks four comments in advance.

1) Don't use a synthetic from the first start is good advice, but it has nothing to do with ring sealing and everything to do with rinsing out the new engine and lines and the expense of the oil.

2 and 3) The 10W-40 will work fine. You don't need an API SM for that application.