Yes, the viscosity changes with multi vis oils........just like it does with single vis oils...........when the temperature changes.
But that DOES NOT mean that 20 W 50 "thickens" to become 50 weight oil at higher temperatures.
The CORRECT interpretation of multi vis oils is (using 20 W 50 as the example) :
The oil has been engineered and certified that at the COLDEST temperatures encountered, the oil will not attain a higher viscosity (thicker) than would a straight 20 weight oil under the same conditions, and that at higher temps (engine operating temp), the oil will not thin more than would a straight 50 weight oil under the same conditions.
The W represents "Winter" in the nomenclature, and 20 W 50 means the oil has the thickening characteristics of a 20 weight oil at cold temperatures while exhibiting the thinning characteristics of a 50 weight oil at higher temperatures.
Whether oil is a single viscosity, or a multi viscosity...........it still thickens when cooled, and thins when heated.
That is the only "magic" in the multi vis rating system............understanding what it really means.
The very common misconception of multi vis oils is that "It thins out to 20 weight when cold and thickens to 50 weight when hot". Wrong, wrong, wrong!
If what some folks "think" was true, actually WAS true.............it would be recommended that you "drain your oil while it is cold and thin"......!

This post was edited by mownie on Sat, Jun 15, 13 at 14:01

Thanks mownie, if I read your dissertation correctly, I'm right in that at operating temp, "thin" 50 has a higher viscosity than "thin" 20 and I shouldn't use it.

Looked at the hydraulic oil in the barn and it's labeled with an "ISO" number? OK, found a site that translates the ISO number to SAE and what I have is the "equivalent" of 20, so I'm good to go.

Ev

Here is a link that might be useful: ISO to SAE chart

Not exactly Ev.
If you look at the chart (in your link) and pick out the viscosity index (using the "Centistokes" scale) you will note that 20W weight oil is numerated as 68 at a temp of 40C/104F. This is the coolish end of the viscosity testing range used in that research.
Now, note that for 50 weight oil at 40C/104F, the number is 220.
220 is quite thick compared to 68 and would likely cause cavitation (at least briefly) in a pump designed for less viscous oil.
Now, look at the upper end of their test parameters.
20W weight oil has a number of 8.7 at 100C/212F.
50 weight oil has a number of 19.4 at 100C/212F.
The focal point of all this is: Your old manual states that it is recommended to use 20W oil, and looking at the chart........that oil will THIN OUT to 8.7 at the high end of test scale. So the operating range for 20W goes from a cool 68 to a hot 8.7.
The implication here is that oil exhibiting a THINNING to not less than 8.7......................is suitable to be used in the trans.
And.........the trans will be happy and well cared for operating anywhere inside the scale range of 68 to 8.7

Now, look at the 50 weight oil on the hot end of the scale. At 100C/212F it thins out to 19.4. That's quite a bit THINNER than 20W at 40C/104F.......but is well above the 8.7 of the 20W when hot.

Because multi viscosity oils are engineered with "pour point depressants" so that they DO NOT thicken as much when cold, you can ignore the low temp number (220) given on the chart for the 50 weight (single vis) oil IF YOU ARE USING 20 W 50 multi viscosity oil.
The pour point depressants used in its formulation prevent this oil from thickening to greater than 68, which represents the low temp value for 20W single viscosity oil.
Essentially, 20 W 50 oil expresses a viscosity range of 68 to 19.4 when used at temps shown in the test chart.
When used in an ENGINE, it means you get the cold starting benefits of a 20W oil and the hot running benefits of a 50 weight oil.
20 W 50 will be perfect for your tranny.

OK, 20W50 it is!! Why is a simple thing like changing the oil in the tranny giving me a headache? :-)

thanks again,

Ev

Because you ain't been a mechanic for 45 years :^)