A couple of observations about the functions of engine oil;
Oil is used to lower friction, maintaining a film on bearing surfaces is obvious, removing heat from the bearings is perhaps less so. Excessive heat will break down oil, and may cause bearing clearances to close to the extent that oil might not be able to form a film, or expand to allow hammering. A thinner oil will remove heat better than a thicker one, in a perfect world the oil in your engine would be just thick enough, or with enough viscosity to be strictly accurate to maintain a lubricating film and shift as much heat as possible.
Different engine designs have different lubrication requirements, one size most definitely doesn't fit all, although in the real world a quality gasoline engine oil will be fine for most modern gasoline engines, but a flat tappet engine will need zinc additive, a pre war L head loose tolerance engine won't like modern 5w30 oil either.
Oil breaks down over time, and multigrade oils in particular suffer from shear degradation, for example 5w40 will behave like 15w30 if you don't bother to change it for 15,00 miles or so. Multigrades with closer viscosity indexes suffer less than oils with wider spreads, 10w30 staying in grade longer than 5w50 all else being equal.
I know nearly all users of this forum are probably very conscientious about changing oil as well as other aspects of proper maintenance so I'm preaching to the choir here. However over years of maintaining cars for a living I have found using oils recommended by the makers has worked just fine for long and reliable engine life. The difficulty anybody in the old car hobby has is getting lubricants suitable for outdated engine designs, from flat tappet engines to ones built 100 years ago.
Roger.
