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A wise man once said that we should think about buying oil in the same way we buy shoes - for which there are an infinite number of variations (style, gender, size, width fitting etc) and "one size fits all" definitely doesn't apply.
That said, no engine manufacturer produces their own oil. Personally, I use Shell Rimula which is very well regarded and quite possibly a better spec than Volvo's own according to an ex-hydrocarbon industry scientist I was talking to a while back. It is also considerably cheaper, but its specification is compliant with VDS-3, which is all you really need to know.
I buy to meet or exceed the manufacturer's requirements which is easy as they state API CD 15W40, changed annually or every 200 hours.
CD has been superseded several times, but since later oils (CF, CG, CH, CI..) are backwards compatible it really isn't too hard to find good alternatives.
To my understanding Marine oils have a different additive package to better meet the potential conditions in the marine environment (perhaps more tolerant to water, condensation, sitting for longer periods) and since they cost close to the same, I'm happy to buy these.
Neither in marine nor car warranty terms have I seen that the use of non manufacturer-branded lubes would void the warranty. The crucial point is ensure 'exactly meet or exceed the manufacturer's requirements' as manufacturers of course will look for ways to reject claims.