My take on this is that these older charts are based on dino oils. With dino oils, the bottom number is the base stock used for the final product. Viscosity improvers are added to bring up the 100*C rating. With age, the viscosity improvers depleat, leaving you with a final product that is closer to the original lighter base stock oil. These oils then become too thin for heavy or high speed sustained use. The industry is probably being conservative in derating the oil because of this.
Meh. You assume “synthetic” is a magic elixir changing everything.
In general the W number represents the base stock viscosity
grade based on oil production technology pre-WWII. Since then much has been learned about which molecules are useful to lessen the viscosity
decrease with temperature. Synthetic manufacturing methods makes selection of desired molecules easier. Synthetic manufacturing has helped identify useful molecules.
Viscosity Improvers have always been synthetic compounds which have been found to lessen the viscosity
decrease with temperature
increase when mixed with base oil. As K7LN says VI compounds are weaker than base oil.
The 2nd number in the viscosity
grade has nothing to do with the weather. It represents the viscosity of that pre-WWII oil at 100°C. Be ware “40” at 100°C is thinner than 5W at it’s rated temperature. As temperature increases 5W-40 gets thinner, not thicker.
Nothing about synthetic or conventional says anything absolute about VI, how much, or even if any is used. Is not listed on the bottle. SAE/API, JASO, ACEA, etc, write very few standards stipulation how goals must be accomplished, mostly just stipulate the goals. They don’t say how to achieve viscosity grades. Don’t say what additives to use, but will limit how much of certain additives are allowed.
Life of multigrade oil is usually determined by how long it stays in grade. How long before the 40 becomes a 30. An oil with more VI additives
probably will not stay in grade as long as another which required fewer to achieve same grade. There are extended drain standards which require viscosity to stay in grade plus longer term acid neutralization and other things. This is not an automatic property of synthetic.
Better base oils are required when additives are limited.
The charts cited do not make sense. At 100°C viscosity of 5W-40, 10W-40, and 15W-40 should be indistinguishable.
The W grades are based on the temperature the oil thickens to a value some deem too difficult to pump through the engine. 20W reaches that viscosity at -20°C. Then every 5 steps 5°C. So 15W is -25°C. 10W is -30°C. 5W us -35°C. 5W is the same viscosity at -35°C as 15W at -25°C. The actual lowest usable temperature is determined by the engine manufacturer.
