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TT recommends 5w-50 oil in their low end transmissions and the chart below shows how motor oil compares to gear oil. Notice that the low number Winter viscosity oils [ 0W,5W,10W,15W] still correspond to basically 75 wt gear lube and the 50 side [ the viscosity flow at 210 degrees ] of the motor oil is equivalent to 90 wt gear oil.

From Bob the oil guy's site

ow do all of these viscosity grades seem easier to understand? Okay, maybe not yet, so Table 5, a viscosity equivalent chart, provides a comparative illustration of all of the grades shown in tables 1 through 4. For example, the chart indicates that an SAE 50 engine oil and an SAE 90 gear oil are the same viscosity. This might surprise you if you think that gear oil is always thicker than engine oil. However, as Table 5 shows, they are nearly equivalent.

Viscosity Charts

http://www.bobistheoilguy.com/viscosity-charts/
 

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Here is an interesting article about using motor oil in hydraulic systems. A hydrostatic transmission is a hydraulic system that also has a gear train in it. Gear trains are murder on the long chain molecules in motor oil. The last paragraph tells you what happens to the oil in your T40/K46 if you never change it. The viscosity deteriorates due to shearing and the lubrication properties deteriorate which kills all the critical close fit tolerances in the motor and pump rotors. Also the aluminum surfaces that the rotors bear against are subject to greater wear due to the old oil. Then your mower won't go up a slope when it gets hot. Since the tolerances are now destroyed changing the oil makes little improvement.

http://www.machinerylubrication.com/Read/29715/hydraulic-engine-oils
 

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TT recommends 5w-50 oil in their low end transmissions and the chart below shows how motor oil compares to gear oil. Notice that the low number Winter viscosity oils [ 0W,5W,10W,15W] still correspond to basically 75 wt gear lube and the 50 side [ the viscosity flow at 210 degrees ] of the motor oil is equivalent to 90 wt gear oil.

From Bob the oil guy's site

ow do all of these viscosity grades seem easier to understand? Okay, maybe not yet, so Table 5, a viscosity equivalent chart, provides a comparative illustration of all of the grades shown in tables 1 through 4. For example, the chart indicates that an SAE 50 engine oil and an SAE 90 gear oil are the same viscosity. This might surprise you if you think that gear oil is always thicker than engine oil. However, as Table 5 shows, they are nearly equivalent.

Viscosity Charts

http://www.bobistheoilguy.com/viscosity-charts/
Thanks Ray, I had a look and well...........do you ever watch "Big Bang Theory" ?

This strikes me as Sheldon's territory.:fing20:
 

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Discussion Starter #5

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I just bought some Hy-Gard trans fluid the other day to change the fluid soon in my X500:thThumbsU
 

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Explains again why oils and greases are formulated for specific applications. Most people ignore the research. Always leaning to a heavier oil when the manufacturer says a thinner one.

slomo
 

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I note that the above articles are aimed at industrial usage of hydraulic/lubrication fluids where fluid change intervals exceed the normal life expectancy of most lawn and garden tractors and for the most part are in use at ambient temperatures above the freezing point where 90W gear lube is sludge and will totally lock up a manual transmission at -35°.

The hydro engineers recommend fluids that will work correctly in their products under all circumstances and fluid change intervals are aimed at getting the maximum realistic life.

Picture the cost to a manufacturer that has to change out the fluids of many hydraulic systems at intervals of less than 2 weeks (300 hours) as we do with our hydros.
 
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