Don't know about the Valvoline, "it's a one size fits all" no idea if it will cause problems.
Engine oil, your engine used 30w full synthetic when new. Volvo quit making it and now recommends Volvo Penta 10W-40 4-Stroke Synthetic Engine Oil. If you want to keep using the 30w, Sierra SAE30 4-Stroke Synthetic Marine Oil is available.
I try and stay away from auto oil, It has additives that aren't needed for your engine, is designed to protect engines that operate very different from your engine and doesn't have the additives that your engine needs.
Not sure why you want to flush it? But if you do I recommend a NMMA rated oil only.
You can use any oil you want, it's your boat. So much depends on how you use the boat as to what's best. Will auto stuff work? Ya probably. Will it work as good as a marine rated oil? Probably not.
Engine oil, your engine used 30w full synthetic when new. Volvo quit making it and now recommends Volvo Penta 10W-40 4-Stroke Synthetic Engine Oil. If you want to keep using the 30w, Sierra SAE30 4-Stroke Synthetic Marine Oil is available.
I try and stay away from auto oil, It has additives that aren't needed for your engine, is designed to protect engines that operate very different from your engine and doesn't have the additives that your engine needs.
Not sure why you want to flush it? But if you do I recommend a NMMA rated oil only.
You can use any oil you want, it's your boat. So much depends on how you use the boat as to what's best. Will auto stuff work? Ya probably. Will it work as good as a marine rated oil? Probably not.