"Any name brand 15w40 should fit the bill. No reason to use 20w50 over 15w40 unless your trying to slow consumption. The engine obviously has a large range for oil choices. 15w40 is just so stout and readily available it's hard to ignore."
+1
Any of the major 15W40 oils (Delo, Delvac, Pennz Long-Life, etc.) are real champs in these kinds of applications. Readily available, affordable, shear stable and with an almost-ideal additive package for dealing with moisture and other contaminants getting into the oil.
To be more specific, with marine engines you have a high-moisture environment. Moisture + byproducts of combustion = acids attacking the engine. HDEO oils have the best add-packs to combat this.
20W50 are usually not as shear stable and don't have nearly the add-pack. It won't thin out to a 30 weight or below ... but all that VII that's broken down could form sludge. It certainly isn't helping protect your engine.
I don't know enough about Mercruiser's 25W40 oils. However, I have yet to see a manufacturer-branded oil that impresses ... and I'm talking about any internal combustion engine application on land, in the water or air. You typically get a higher price but mediocre-at-best performance.
+1
Any of the major 15W40 oils (Delo, Delvac, Pennz Long-Life, etc.) are real champs in these kinds of applications. Readily available, affordable, shear stable and with an almost-ideal additive package for dealing with moisture and other contaminants getting into the oil.
To be more specific, with marine engines you have a high-moisture environment. Moisture + byproducts of combustion = acids attacking the engine. HDEO oils have the best add-packs to combat this.
20W50 are usually not as shear stable and don't have nearly the add-pack. It won't thin out to a 30 weight or below ... but all that VII that's broken down could form sludge. It certainly isn't helping protect your engine.
I don't know enough about Mercruiser's 25W40 oils. However, I have yet to see a manufacturer-branded oil that impresses ... and I'm talking about any internal combustion engine application on land, in the water or air. You typically get a higher price but mediocre-at-best performance.