15w-40 is commonly used but a 3.5+ hths euro will do good. I haven't heard of anyone having issues with rust when using non marine oil. I presume the first marine oils did have corrosion inhibiting additives because regular oil didn't back then but that was so long ago and nowadays all modern motor oils have corrosion inhibitors. Maybe marine has a bit more but I don't think it matters. I've yet to see these engines rust internally when using 15w-40 or anything else.