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Merc and Volvo's recommendations have always been slightly different but the thing to keep in mind is that they both use essentially the same engines from GM. Merc now recommends their 25/40 which is a combination of those 2 straight weights with no VI improvers according to Merc. But they also recommend straight 30 under 50*F and straight 40 above. Volvo has recommended 20w/50 in the past, they were selling a straight 30 syn until last year and then they changed to a 10W40 syn. OMC pretty much always recommended a straight 30. Crusader, who is one of the best respected marinizers of small block Chevys, has been recommending the 15w-40.

Graham the preference for straight weights most likely came from the fact that conventional multi vis oils have to use viscosity index improvers to get the oil to span the range between the hot and cold viscosity levels. These additives can cause trouble over time, when the break down the oil can shear down to a lower viscosity. And they can also cause piston ring sticking. Syn oils do not need VI improvers so that is a benefit. Straight conventional oils do not use VI improvers either and thats why they were preferred in the time before syn oils were used in marine engines.

I would not use a conventional multi vis oil in a marine inboard, I feel that they can thin out under sustained high rpm use. I'd either use the Merc 25/40, straight 30 or 40 depending on your temps in summer, or an HD 15w/40 like Delo 400 or Shell Rotella T.

With straight 40 my engine has a cold idle oil pressure of 40 and a hot idle oil pressure of 20. I'm sure those numbers would be lower if I used a multi vis conventional oil. I have never had any trouble with starting it at temps as low as 45*F with the straight weight oil.

Graham back in the day when many of us college kids drove air cooled VW Beetles, the preferred oils were straight weights for the same reason, those engines ran hot and multi vis oils would break down. I used straight 30 and 40 in mine depending how hot it was in summer and a 20w in winter.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

I run chevron delo 400 15w40 in my boat. I also use it in my truck (96 F250 powerstroke). Never had a problem. I figure if the big rigs that run around here hauling 100,000+ lbs up and down the mountain and get serviced every 10,000 miles I think it will work in my boat. I understand that boats are always under load but just can't see it being any different than a big truck. Oil is oil is oil use a good quality brand and change it every year.

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'96 200 Horizon 5.8l VP/SX