In my 89' Conbrio, I used Castrol 20W-50 conventional oil. I had the boat for 24 years and logged over 1500+ hours on the 351 Windsor motor. I took the valve covers off in 2015 to replace the gaskets and the heads look factory new. I only ran my boat in 75°+ weather. I would NOT use this in colder climates. My 2004 Supra(350 GM) called for Pennzoil 15W-40 in the manual, which is generally used in heavy duty service and diesel engines. I am using the Shell Rotella and have had no issues. I used a Mobil 1 filter as it is better constructed than most filters and, according to testing does filter better. When I slalom skied, the engine was running between 3000-3500 RPM, which I don't believe 10W oil offers enough protection. Automotive engines don't see the sustained higher RPM that most boats do.
Just my 2cents.