This from a GM powertrain engineer specializing in engine design. I thought it might be sort of thought provoking for those of us looking for the "perfect oil" for their beloved classic car.
"Older engines had rubbing element tappets with relatively high contact stresses that use up the antiwear additives in the oil, spur gear oil pumps that shear the oil viscosity down, distributor drive gears for the distributor and oil pump that shear the oil as well as use up the antiwear additives. Things like rocker arms that pivot on bushing style bearings or spherical supports also use up the antiwear properties of the oil. Older engines had PCV systems that were often not nearly as efficient at purging the crank case. Carbureted engines put much more fuel into the oil on cold starts due to the relative inaccuracy of fuel delivery cold and the greater amount of fuel needed to be delivered due to "wet" intake manifolds.
All of these factors shear the oil viscosity down faster, use up the antiwear additive in the oil (the ZDP) faster and contaminate the oil more requiring more frequent oil changes. Similarily, these engines are much more dependent on "good" oil to live as the various rubbing elements need high amounts of ZDP in the oil to live and the engines will fail sooner if the oil is not changed more frequently and are much more sensitive to the quality of the oil used.
Modern engines have eliminated most all of the rubbing elements inside the engine by use of roller tappets, roller rocker arms, etc... The elimination of the distributor negates the need for a distributor drive gear eliminating that source of oil degradation. Mounting the oil pump drive off the crank and switching to gerotor style oil pumps also negates the need for the distributor gear and the gerotor pumps do not shear the oil viscosity nearly as bad. Port fuel injected engines dramatically minimize the amount of oil contamination during cold starting and short trips as the amount of extra fuel needed on cold starts is minimized and good driveability is maintained during warmup with little extra fuel due to the "dry" manifold designs.
Similarily, the modern engines do not "wear out" the oil nearly as quickly and they are not as dependent on oil quality to survive. Reduced levels of the antiwear compounds are fine since there are few areas in the engine as dependent on them as the older flat follower engines with distributor gears were. Also, with all the rolling element parts the current engines can easily live on thinner oils which are good for fuel economy and do not jeapordize engine life as the oil ages and thins."
"Older engines had rubbing element tappets with relatively high contact stresses that use up the antiwear additives in the oil, spur gear oil pumps that shear the oil viscosity down, distributor drive gears for the distributor and oil pump that shear the oil as well as use up the antiwear additives. Things like rocker arms that pivot on bushing style bearings or spherical supports also use up the antiwear properties of the oil. Older engines had PCV systems that were often not nearly as efficient at purging the crank case. Carbureted engines put much more fuel into the oil on cold starts due to the relative inaccuracy of fuel delivery cold and the greater amount of fuel needed to be delivered due to "wet" intake manifolds.
All of these factors shear the oil viscosity down faster, use up the antiwear additive in the oil (the ZDP) faster and contaminate the oil more requiring more frequent oil changes. Similarily, these engines are much more dependent on "good" oil to live as the various rubbing elements need high amounts of ZDP in the oil to live and the engines will fail sooner if the oil is not changed more frequently and are much more sensitive to the quality of the oil used.
Modern engines have eliminated most all of the rubbing elements inside the engine by use of roller tappets, roller rocker arms, etc... The elimination of the distributor negates the need for a distributor drive gear eliminating that source of oil degradation. Mounting the oil pump drive off the crank and switching to gerotor style oil pumps also negates the need for the distributor gear and the gerotor pumps do not shear the oil viscosity nearly as bad. Port fuel injected engines dramatically minimize the amount of oil contamination during cold starting and short trips as the amount of extra fuel needed on cold starts is minimized and good driveability is maintained during warmup with little extra fuel due to the "dry" manifold designs.
Similarily, the modern engines do not "wear out" the oil nearly as quickly and they are not as dependent on oil quality to survive. Reduced levels of the antiwear compounds are fine since there are few areas in the engine as dependent on them as the older flat follower engines with distributor gears were. Also, with all the rolling element parts the current engines can easily live on thinner oils which are good for fuel economy and do not jeapordize engine life as the oil ages and thins."
