Hi,
Almost since I bought our 2003 Prius, I have been fascinated by Prius transaxle oil. The NHW11 model used Type T-IV but the subsequent models used Type WS. I soon learned that changing the transmission oil significantly reduced cold temperature mechanical drag but I also learned Amsoil ATF was not the right answer. Currently, I'm using Type WS because as Type T-IV wears, its viscosity approaches that of Type WS. But I just finished reading a handbook that explains what has been going on:
The Practical Handbook of Machinery Lubrication, 3d Edition, Llyod O. (Tex) Leugner, Noria Publishing, 1328 East 43rd Court, Tulsa, OK 74105, $70+shipping
Tex provides a combination of theory and practical knowledge about lubrication without the sales malarkey found at different lubrication company web sites. Early on, he describes the three forms of lubrication: (1) boundary layer; (2) hydrodynamic, and; (3) elastohydrodynamic. Boundary layer is the typical lubrication film between the surface imperfections. But Tex goes on to discuss in depth the various solid coatings such a molybdenum, boron nitride and teflon. For example, suspended in the oil they are nearly useless but applied as a surface treatment before assembly, they can provide valuable protection during start-up.
Hydrodynamic refers to how as the velocity differences between moving parts increase, the oil creates high pressures to keep the parts from rubbing. The oil literally suspends the moving parts. But Tex also discusses the start-up wear that happens until the parts actually reach operating speed and mitigation systems such as oil pressure before load. This is how our Prius engines start with extended spin-up before the engine begins feeding fuel.
Elastohydrodynamic is new to me but at high pressures and speeds, the oil behaves as a solid, nearly perfect separator of moving parts. The oil viscosity changes but becomes a very efficient, very low friction surface insulating the two parts. For years, I'd noticed that as NHW11 speeds approached 65 mph, the vehicle efficiency improved even more than engine efficiency would dictate ... a 'sweet spot' at 65 mph. It may be we are seeing parts of the car operating in this most efficient, low mechanical drag region.
One of the lessons learned about Amsoil ATF is it 'eats yellow metal' and in particular, the copper measurements went rapidly too high. I caught it and switched to Type WS. However, Tex speaks about a copper test, the right answer when looking at any alternative lubricant for the NHW11 transaxle.
The last bit of the puzzle came from Tex explaining how polymers are added to base oil to extend the viscosity and that the base oil does not 'wear out.' However, these viscosity range improving, polymers are subject to 'shear down.' This explains how the Type T-IV approaches Type WS viscosity over time. In fact, Tex points out that oil doesn't wear as much as the additives are exhausted and it gets loaded with wear material. This certainly is consistent with the oil testing results.
At $70, this handbook is a little more expensive than popular books but the knowledge it provides, the insights, makes it well worth it to the curious. It is free from marketing FUD making it a useful guide for any technician or engineer who really wants to understand lubrication.
Bob Wilson