We have all known that overfilling enough to cause foaming is bad for engines. Here is an article with one expanation of how the air bubbles can cause oil aging and failure.
From "Oil Analysis Basics."
Lubricants can thermally degrade for a variety of reasons and causes. Unlike oxidation, thermal failure can occur in new lubricants with healthy additive packages. However, many of the symptoms of oxidation are also symptoms of thermal degradation.
One of the most common causes of thermal failure in hydraulic fluids and some lubricating oils relates to aeration, i.e., entrained air bubbles. These bubbles can become rapidly compressed in hydraulic pumps and in the squeeze zones of bearings. This results in extremely high localized temperatures (adiabatic compression) leading to sudden formation of carbon fines. Hot surface carbonization is another form of thermal failure. When an oil thermally degrades, problems associated with sludge, varnish, deposits, viscosity change and additive decomposition will often occur.
From "Oil Analysis Basics."
Lubricants can thermally degrade for a variety of reasons and causes. Unlike oxidation, thermal failure can occur in new lubricants with healthy additive packages. However, many of the symptoms of oxidation are also symptoms of thermal degradation.
One of the most common causes of thermal failure in hydraulic fluids and some lubricating oils relates to aeration, i.e., entrained air bubbles. These bubbles can become rapidly compressed in hydraulic pumps and in the squeeze zones of bearings. This results in extremely high localized temperatures (adiabatic compression) leading to sudden formation of carbon fines. Hot surface carbonization is another form of thermal failure. When an oil thermally degrades, problems associated with sludge, varnish, deposits, viscosity change and additive decomposition will often occur.
