Agreed, I've heard that higher vis oil is actually not advised, but never saw evidence of supposed damage it could cause. If Volvo said syn 10w-40 then so be it. I use the conventional Merc 25/40 in mine. I can tell you this....I had salt water in the engine back in '16 took me a while to figure out what was happening (blown HGs) but when I took it apart, very little to no corrosion on the pushrods, rocker arms, rocker nuts, studs, etc. When I pulled out the hydraulic roller lifters one by one to check, the lifters looked new, no corrosion no wear, same with the cam lobes. So I believe their claims of better corrosion resistance than standard oil....