Originally Posted By: Shannow
Originally Posted By: fdcg27
Even the straight grades aren't really straight.
The basestock will have its own VI, so even a straight grade will be thinner at cold start than it's SAE viscosity grade might lead you to think.
In your climate, you could probably run a 20W-20 winters and a 30 in summers with no ill effects in most engines.
To be a "straight" weight, the lubricant has to be "Newtonian", and typically/always devoid of polymeric thickeners.
So, regardless of VI, a "straight" weight will have an identical viscosity regardless of shear rate for a given temperature...if your crankshaft is spinning 1,000 RPM, 3,000 RPM, or 20,000 RPM, at 100C, your 11cst 30 will be 11cst at every rpm increment.
Until multigrades were developed, the concept of "HTHS" was irrelevant (more so "not yet needed"), the normal viscosity classifications applied, and worked everywhere.
Multigrades with polymeric VII are non newtonian, and have different viscosities at the same temperature depending on the shear rate of the oil...the viscosity in your bearing at 1,000, 3,000, or 20,000 will be different.
That's why they introduced the HTHS concept, and requirement into J300...in the words of SAE
Quote:
To insure that polymer-containing oils do not create a situation in which the viscosity of the oil decreases to less than a specified limit, minimum values of HTHS viscosity are assigned to each of the non-W viscosity grades in Table 1. A
special situation exists regarding the SAE 40 grade. Historically, SAE 0W-40, 5W-40, and 10W-40 oils have been used
primarily in light-duty engines. These multigrade SAE 40 oils must meet a minimum HTHS viscosity limit of 3.5 mPa·s.
In contrast, SAE 15W-40, 20W-40, 25W-40, and 40 oils have typically been used in heavy-duty engines. The manufacturers of such engines have required HTHS viscosity limits consistent with good engine durability in high-load, severe service applications. Thus, SAE 15W-40, 20W-40, 25W-40, and single-grade 40 oils must meet a minimum HTHS viscosity limit of 3.7 mPa·s.
http://paservice.it/wp-content/uploads/2013/06/J300_201304.pdf
Out of interest, an SAE30 "newtonian" oil of around 10-11cst, KV100 typically will be around the 3.5-4cst at 150C...the original 2.9 for the 0, 5, and 10W-40s show how badly VII wasn't so good when developed...the current requirement for 3.5 for those same grades shows just how much modern VII have improved in recent years, in being able to meet those specs in (allegedly) every oil that you see holding the SAE ratings.
A High VI "straight weight", like Amsoil 30 can claim 10W, as it's cold temperature performance can meet those requirements...but it's still newtonian and can claim "straight"
Very well stated. Thank you