Need advice: 32 vs 46

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My 2005 Unimog U500NA owners manual calls for 10W hydraulic oil in the hydraulic system. The hydraulic system powers the rarely used winches, but also the radiator cooling fans. Low stress but constant use.
Mobil’s 10W hydraulic oil has a vis40 of 37.7. I live in Anchorage which can rarely get below -10F. Don’t use truck much in winter, but winches might be used to pull someone out of ditch.
Most use is in warmish places; may be in Kalahari 2022-3 southern summer.
System takes 45L. Right now has mainly Delvac 1 LE 5W30 which has vis40~70 (close to 68).
Right now I have 2 unopened buckets of Mobil DTE 10 32 viscosity. Very good stuff with high VI.
Should I return it for same stuff 46 viscosity? I am hoping dropping viscosity to the proper range would improve fuel economy 1 or even 2%.
PS local jobber doesn’t stock the 10W stuff and it has comparatively crummy VI.
 
Wow, 11 gallons of fluid to run cooling fans?

Seems like a good opportunity to look into how you might convert over to electric. To me, that much fluid implies a big system and a big reservoir, and thus more opportunity for damage, failed seals, lost fluid, challenges to top back up to full... and loss of a winch is one thing, a fan is another...

Not the answer you wanted to hear Im sure...

Im somewhat confused. 10w hydraulic oils are often marketed as iso 32. You have two buckets (10 gallons) Of 10/32 fluid thst you think has good specs. What’s the issue? If Daimler says thst is appropriate for all conditions, that’s the stuff to use. Might want to check the bevo sheet for the spec fluid to see what’s on there. If Daimler only specs thst fluid for frigid temperatures, and you want to use it someplace else, then what does Daimler spec for hot temperatures?
 
11 gal for fans, 2 winches and if needed 3.6-4kw hydraulic 120/240 generator. The volume is so great because some gadgets like giant snowbank eaters have a big drawdown if plugged in empty.
Thanks for the electric fan thought, but I am NOT messing with a system on a $125k chassis that’s worked perfectly for 100k miles/14 years.
Bevo: good idea, but they are strangely silent on Unimog hydraulic oil. Only the USA owners manual.
But I just checked UK owners manual. Same stuff 10W motor oil/228.0. Seemingly not a very rigorous specification.
Mobil 10W is vis100=6.1, vis40=37.7, VI 107, pour point only -30C.
The DTE 10 Excel 32 is vis100=6.63, vis 40=32, VI 164, pour point -49C.
And the DTE meets a ton of hydraulic mfg specs and the 10W mentions only 1 Eaton spec.
But…the 10W does meet API CC and my engine sump holds 29.6L. My fans would still run with 16L in the reservoir. Maybe not.
I think I will stick with the DTE 32, thanks for helping.
 
My 2005 Unimog U500NA owners manual calls for 10W hydraulic oil in the hydraulic system. The hydraulic system powers the rarely used winches, but also the radiator cooling fans. Low stress but constant use.
Mobil’s 10W hydraulic oil has a vis40 of 37.7. I live in Anchorage which can rarely get below -10F. Don’t use truck much in winter, but winches might be used to pull someone out of ditch.
Most use is in warmish places; may be in Kalahari 2022-3 southern summer.
System takes 45L. Right now has mainly Delvac 1 LE 5W30 which has vis40~70 (close to 68).
Right now I have 2 unopened buckets of Mobil DTE 10 32 viscosity. Very good stuff with high VI.
Should I return it for same stuff 46 viscosity? I am hoping dropping viscosity to the proper range would improve fuel economy 1 or even 2%.
PS local jobber doesn’t stock the 10W stuff and it has comparatively crummy VI.

The best answer is to understand the minimum viscosity requirements for the components (pumps/motors) in your system, and the temperature extremes you expect to be working in. I would advise NOT to use polymer enhanced (high VI) fluid in a hydraulic system, as they tend to shear down quickly in hydraulic pumps and motors.
 
I thought of that also. But…
1) >99.9% of my hours are turning 1-2 hydrostatic fans that consume minimal power and are at a small % of max rated rpm except when ascending long highway grades.
2) DTE 10 Excel is advertised by Mobil as a ”premium product”; why would they sell it, and why would it have a jillion mfg approvals, if shearing were a big problem with it?
3) I will re-enquire if the jobber has the 10W, but the idea of trying to run a winch without idling for 1-2 hours first in -10F worries me.
4) the Mobil PDS says “The shear stable high viscosity index allows for….”.
Again, appreciate these thoughts.
 
For extreme cold 5w10 or 5w20 SAE or 10w for all season was recommended but that is Euro spec I have no idea how the 5Wxx would work where you are but I suspect it may be a good fit.
 
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The 2003-6 U500NA (they only sold ~300) has some electrical differences (hybrid 12v/24v), pneumatic also (8.5 bar instead of 18.5), but hydraulic stuff is identical. Real sure, I think I know these things pretty well.
The 5W30 has worked well but it’s time to change it and I’d appreciate 0.1-0.2 mpg increase.
Maybe I’ll put the DTE 32 in and check a sample for vis40 after Africa.
I have only 10 gal so might have to top up with some Chevron Rykon AW32 I have leftover from something.
 
2) DTE 10 Excel is advertised by Mobil as a ”premium product”; why would they sell it, and why would it have a jillion mfg approvals, if shearing were a big problem with it?
Echoing DriveHard's advice to avoid high VI fluids in regular used hydraulics where practical. I, unlike him, like them in occasionally used systems such as aircraft hanger doors with a 200 VI ATF which need to work from -40F/C to 130F ambient. The DTE Excel fluid datasheet is at https://www.mobil.com/en-us/industrial/pds/gl-xx-mobil-dte-10-excel-series

Note that DTE 10 and DTE 10 Excel are different products; DTE 10 is not a high VI fluid. The Excel variant is moderately high. I expect the Excel variant to be higher cost. I personally feel (not to step on any BITOG sponsor's toes) that Mobil DTE products are the best mainline hydraulic fluids out there and would have no qualms using one in the appropriate temperature range.

Considering economy, I'd probably recommend DTE 10 Excel 32 for you. SLIGHT slippage during your winch operations wouldn't seem to be enough to move to the 46 grade. Indeed, without knowing components, I MIGHT even use DTE 10 Excel 22 for lower pumping losses with your 100% duty loads.

EDIT!!!!! It looks like the "standard" DTE 10 series with which I was familiar with is no longer. DTE 10 Excel is a new moderate VI fluid.
 
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