Transmission Oil Temperature
- Thread starter Ruth
- Start date
cadillac512
Well-known member
The way a multi vis oil works is when cold it behaves like the lower number (like a cold 85) and as it warms up the additive package keeps it from thinning as much so it ends up behaving like a ("hot" 140) when fully warm. It's an attempt at keeping the viscosity level over the operating temp range.
If your Mitchell is operated at moderate temps (no winter temps) I'd say straight 140 will be fine. The shifting will be slow if it gets very cold though.
If your Mitchell is operated at moderate temps (no winter temps) I'd say straight 140 will be fine. The shifting will be slow if it gets very cold though.
nkaminar
Well-known member
To expand on what Cadillac said, the viscosity of a standard oil has a fairly steep slope. It declines a lot with increasing temperature. The viscosity of any multi-grade oil declines with temperature by not as steeply. So the difference is really in the slope of the viscosity curve with temperature. There is no sudden change from one viscosity to the other. Google "85W-140 viscosity curve" and click on images to see the curves for different oils.
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Jim Brierley
Well-known member
I wouldn't run 140, if you don't like 85-140, use 85-90. Thinner oils reach all parts quicker than 140, and is probably more available.
It is not that I dislike the 85W-140, it is very hard to find GL-4 85W-140 but GL-4 140 is readily available. I have been told that GL-5 is not good for "yellow parts" in the transmission. Both Mitchells have brass synchro rings.Most 85W-140 oils are GL-5.
The second part is Does the 85W-140 oil actually obtain a temperature to act like 140? I have seen that it becomes 140 at 210°F, which I don't believe my transmission and O/D run at. I'll have to check that next time after a long drive.
Probably nothing to be concerned with, just looking to be educated...
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Werner
Well-known member
Guten Nachmittag Ruth.
It is true that GL-5 is less suitable for our gearboxes due to its additives.
You can use GL-3 too in addition to GL-4.
If you want to compare the temperature/viscosity characteristics of a multigrade 85W-140 oil versus a single-grade 140 oil, try googling "V/T diagram"; you can quickly see there just how much the viscosity of single-grade oils changes with temperature.
Significant changes in viscosity have a major impact on gear shifting. The time required for the gears to 'synchronize' (during the pause between shifts) increases as the oil thins out, meaning you constantly have to adjust the timing of your shifts.
It is true that GL-5 is less suitable for our gearboxes due to its additives.
You can use GL-3 too in addition to GL-4.
If you want to compare the temperature/viscosity characteristics of a multigrade 85W-140 oil versus a single-grade 140 oil, try googling "V/T diagram"; you can quickly see there just how much the viscosity of single-grade oils changes with temperature.
Significant changes in viscosity have a major impact on gear shifting. The time required for the gears to 'synchronize' (during the pause between shifts) increases as the oil thins out, meaning you constantly have to adjust the timing of your shifts.
rotorwrench
Well-known member
The viscosity index modifiers are expandable polymers like tiny plastic coils that just look like a fine powder before they are blended into the lubricant base stocks in large mixing vats. After mixing, a person can't really tell the stuff is in there.
The polymers expand when warm and contract when cold. Temperatures for expansion and contraction aren't in a real wide band. It is likely between freezing and around 100 degrees F. At least, that's how it is rated by the API and SAE.
The ratings for different gear pressure figures are based on how much friction load is generated in gear sets and what rpm they operate at. In a slow moving vehicle, rpm and pressures are low enough to use GL1. In medium speed and higher rpm applications, the GL4 is adequate. For high load, high speed, and heavy applications, the extra pressure GL5 is needed. The acidic HP additive is hard on babbitt, phosphor bronze, and brass metal alloys so Medium Pressure GL4 is applicable. Any transmission with a roller sprag type one way clutch also needs low to medium pressure additives.
The polymers expand when warm and contract when cold. Temperatures for expansion and contraction aren't in a real wide band. It is likely between freezing and around 100 degrees F. At least, that's how it is rated by the API and SAE.
The ratings for different gear pressure figures are based on how much friction load is generated in gear sets and what rpm they operate at. In a slow moving vehicle, rpm and pressures are low enough to use GL1. In medium speed and higher rpm applications, the GL4 is adequate. For high load, high speed, and heavy applications, the extra pressure GL5 is needed. The acidic HP additive is hard on babbitt, phosphor bronze, and brass metal alloys so Medium Pressure GL4 is applicable. Any transmission with a roller sprag type one way clutch also needs low to medium pressure additives.
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We have a Mitchell overdrive in our '30 coupe and run 85/90 gear oil in it. Even with that on sub 60º mornings I put the od in neutral and the main in 3rd and let it warm up for a few minutes or the od is very hard to shift. Maybe it is just ours but 140 oil was virtually unusable. Dale
The Master Cylinder
Well-known member
Since the original oil was a single grade and heavier than your GL-4 140, I would guess that your 140 should work OK tho I have never tried it.
That said, Steve Mitchell sez to use GL-5 85W-140 in his transmissions and O/D. Also sez NOT to use 600W from the vendors.
That said, Steve Mitchell sez to use GL-5 85W-140 in his transmissions and O/D. Also sez NOT to use 600W from the vendors.
Bob Bidonde
Well-known member
I have been using GL5 85W140 Gear Oil for many years without issues. A lesser viscosity will effect gear shifting.