Re: whats the deal with straight 30 weight oil recommended for my engine?
Years ago the oil was different also,it had zinc additives which have been removed or extremely reduced to meet emissions in automobiles. The "Marine" oils have the higher zinc,and are FC-W rated(water) where automobile oil is not. See Below
Quaker State engineer Mark Ferner said zinc's extreme pressure qualities aid in reducing piston ring-to-cylinder bore wear at BDC and TDC, where the rotating assembly sees an abrupt change in velocity and direction. Besides its extreme pressure protection, multifunctional zinc also functions as an antioxidant. That is, things exposed to heat and oxygen are subject to oxidation. In the case of a ferrous metal like iron, oxidation causes corrosion, which we see as rust. As applied to motor oil, Ferner said as the "heat accelerates oxygen, the oil starts to oxidize and thicken, changing into other molecular combinations. As the oil thickens, it forms deposits or varnish. In motor oil, the antioxidant additives first sacrifice themselves to prevent this oxidation from occurring, but if they get overwhelmed, the oil eventually turns into a hard, crusty sludge."
In its antioxidant role, zinc fights heat buildup in various hot spots like the piston-to-piston ring interface and the underside of the piston itself. Oil can accumulate in the honing crosshatch marks in the cylinder bore where it is exposed to combustion temperatures as high as 400-450 degrees F, a prime candidate for oxidation if not protected. Today's modern reduced-zinc passenger-car street oils contain other dedicated antioxidants to make up for the loss of zinc.
The potential zinc downside is that in theory, an engine with excessive blow-by could accumulate zinc deposits in the combustion chamber, potentially becoming a detonation magnet. But Ferner maintained, "You would need a fairly good amount of blow-by to really be bad for the chamber. There used to be 1,400-1,500 zinc ppm in [passenger-car] motor oils, about where [Shell] Rotella [diesel-truck] oil is now. We ran those high levels for years and years and never had a problem with chamber deposits building up. Zinc accumulation mainly affects oxygen sensors and catalytic converters, which are very sensitive to the ash that may accumulate on them."
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