Hmmm, I go away for several days and you guys have more fun with viscosity.
Just a few thoughts if there are any particular people involved with this. As with whatever your field of expertise, when folks outside of that field use the terminology you're very familiar with in a "loose" way, you may feel the need to get it back on track. Professionally I've been away from this stuff for almost 20 years, but a lot of what we have been talking about is pretty basic and hasn't changed much in that time. I'd happily admit ignorance if something "new" is outside my experience.
The chart Mrtexas put up is intended as a cross reference tool for viscosity grades only. The word grade is important because, as noted, only the ISO 3348 is a direct reflection of actual product testing resullts. The other 3 are arbitrary numbers assigned by the specification defining bodies. Lets start by defining those.
ISO is the International Standards Organization. They developed this viscosity spec so that no matter where in the world a piece of equipment was manufactured or ended up, the user could find a product to meet the equipment viscosity need. Oil companies around the world have adopted this system of viscosity grading for "industrial" oils such as hydraulic fluids, turbine oils, process oils, and many more applications.
AGMA is the American Gear Manufacturers Assoc. They decided many moons ago to publish their own system of viscosity identification. These are intended mostly for industrial use gear drives, think conveyor belts, elevators, and a host of other apps.
SAE is Society of Automotive Engineers, a U.S. based organization, that defines specifications for all manner of automotive equipment and products. In this case we have the viscosity designations for engine oils and gear oils. BTW, the numbers listed in the gear section are out of date, in 2005 a 110 and 190 grade were added as the automotive industry felt the old grading system offered too wide a range of viscosities for certain applications.
As noted the chart is to be read horizontally to compare similar viscosity grades. That's not to be confused with any two oils of any given grade being the same in performance, other than viscosity. While viscosity, in almost all cases, is the most important characteristic of lubricating oil, additive performance is also an important consideration, and is what will distinguish a motor oil from say a steam cylinder oil, even though they may have the same or very similar viscosity. Most oil companies have a fixed inventory of base oils to build on. For instance, they could/would use the same base oil to make an SAE 40 motor oil as they would to make an SAE 90 gear oil, the difference would be the additive system, viscosity wise they'd be the same.
Just another example: Say you have a non hypoid lubricant, either SAE gear graded or ISO industrial oil (both of which would be fine for spur gears for instance) that met the 140 grading. It wouldn't work as well in a hypoid gear because it lacks the extreme pressure additive needed because of the extra sliding action caused by the high angle contact between the opposing gear teeth of the ring and pinion. The extreme pressure additive (often a sulphur-phosphorous compound) reacts to high temperature spikes from sliding friction and forms an oxide at point of friction to act as a sacrificial wear barrier. That aggressive oxidation function is what is unfavorable to yellow metals such as brass. The oil film is still present, but the additive is there to act when the oil film thickness is diminished by pressure.
BTW, there is no such thing as an SAE gear grade 140w or 250w, minor point, but important if you think that's what you're shopping for and can't find it.
I'm intrigued by the notion that 600w oils "used to be" different/heavier. I'd love to see an old data sheet from back in the day, my bet would be that viscosity then was stated in SSU/SUS, a now outdated system of measuring viscosity. Has someone not familiar with all the viscosity stating variants gotten confused?
If you want to get a little deeper into the woods on this stuff here's a reference:
http://www.tribology-abc.com/abc/viscosity.htm
As stated in the other thread, this is a simplistic commentary (believe it or not
), there are lots of variables when it comes to specifics.