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Discussion Starter #1
Our old 48" Hustler came with 20w50 dino oil in the hydro drive. That was in the 90s. Oil has evolved since then, and finding 20w50 is getting hard and spendy. Oil quality has also gone up as manufacturers ratchet up requirements.

So I made the executive decision - I'm retired from the corporate world of ties and lies - to run 10w30 synthetic in there this time.

I'll keep y'all posted, but expect no issues. Here in Central Texas, we never mow in high heat due to fire risks. Morning mowing when the dew is still out lowers operating temps and the risk of wildfires.
 

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Very bad idea
The surface of the cylinder block & the kidney plates are machined to a specific roughness ( smoothness if you like ) to match the specified oil.
As they get older the roughness smooths out and they leak
As they get older erosion makes grooves & they leak .
Going down to a lower viscosity synthetic oil will render the tranny useless.
 

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Discussion Starter #3
Thanks for sharing.
 

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Very bad idea
The surface of the cylinder block & the kidney plates are machined to a specific roughness ( smoothness if you like ) to match the specified oil.
As they get older the roughness smooths out and they leak
As they get older erosion makes grooves & they leak .
Going down to a lower viscosity synthetic oil will render the tranny useless.
I'm sorry, I have never heard anything like this before.

From rebuilding hydro controllers on one of my walkbehinds, while they have a bazillion versions of a given model, the versions are for different external stuff (shaft sizes/types, control orientation, additional pump for something else), but the main internal bits are all shared between the versions. And then the manufacturer of it recommends a range of fluids for it, but then adds the specific fluid to use is up to the device manufacturer (Hustler in the case of the O.P.), based on how it is to be used (environment, external temps, how it is used).

I've also looked at the parts list for the hydro's my Gravely zeroturn, and it's the same. Lots of variations, but internal bits are shared, and there's no hint of specific parts a specific oil viscosity.
 

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Discussion Starter #5
My small engine guy believes that the heavier oil is specified for models likely to suffer abusive use. We all know that owners tend to be easier on machines than do employees. Skin in the game.

I saw sparks one warm evening, so ever since that we mow after dawn when the dew is still out.
 

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...and to follow up, that "warning" makes no sense, as the viscosity of the specified oil changes with temp, so presumably, 20W50 oil would be fine cold, then stop working as it gets hotter...

My Gravely ZT XL 42, which is sold as a higher-end residential model, also specs 20W50 oil for it's hydro's, but I would expect 10W30 to function just fine (albeit, I did put in 20W50 when I changed their fluids/filters).
 

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I've owned 3 zero turns.
2008 Hustler FasTrac
2013 Hustler Super Z
2020 Snapper Pro s200xt
All 3 specified SAE 20w-50 in the hydraulic system.
Why change oil viscosity from what the owners manual recommends ????
 

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better off with the syn. - That motor is modern enough not just to handle it, but to benefit from the higher quality, ON the other hand, how often do you change that oil? So go with the recommended weight and use the syn..
 

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yes. did not mean to say motor. how often is oil change for hydro drive required?
 

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One thing that slips my mind is that my local farm stores carry Mystic oils in heavy weights for really cheap. I figure my next hydro oil changes in my Cubs will be Mystic syn-blend 15W-50 from Atwood's or Orshelin's. I can get it for $28 for 2 gallons, $3.50/qt.

 

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Discussion Starter #13
One thing that slips my mind is that my local farm stores carry Mystic oils in heavy weights for really cheap. SNIP
Nice. Our local farm stores compete with TSC, so don't stock oil. TSC is hard to beat on lubricants etc. What area are you in?
 

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Nice. Our local farm stores compete with TSC, so don't stock oil. TSC is hard to beat on lubricants etc. What area are you in?
NW Arkansas. How about you?
 

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Dave,
Well you have heard it now
As you should know nothing that is machined is ever perfectly smooth.
Around the 60's a new specification got added to engineering drawings.
It is a little triangle with one side extended so it looks sort of like a ladle .
Read this surface roughness chart if you don't understand the idea
When oil flows over a surface the little furrows create pockets which impeede the free flow of the oil.
Very much like the cross hatching on a cylinder wall .

As you know there is no seal between the cylinder block & the kidney plate ( valve plate to some ) in both hydro pumps or motors.
Some oil will escape because the cylinder block is rotating & the kidney plate is stationary
To work. the oil that escapes from between the plates has to put up more resistance to flow than going through the valve on the pump or motor .
Thus it must "leak out" at a controlled rate which is done by the surface roughness and the clearance between the cylinder base & the kidney plate.
As this distance increases, the hydro wears out and makes the hydro growl that we are all used to.
The growl is the cavitation of the oil escaping between the cylinder & kidney plate.

Now, the thinner the oil the faster it will escape so using a thinner oil will drastically reduce the output power of the hydro .
And at the other end, a thicker oil offers more resistance to the pistons pumping it and being pumped by it .
Thicker oils also add more drag to the large surface area between the cylinder chest & the kidney block.

Thus it is a fine line between working properly and not working at all .
The hydro companies design their units to work best with a specific viscosity of oil .

Next problem is operating temperature.
This can range from well below freezing when shifting snow to well above boiling when cutting grass in the central plains in high summer .

Now JD decided to use 10W 40 because they wanted more sales and more power to be available for shifting snow.
The down side is the hydro wears fast and is generally totally stuffed by 1000 hrs

The units K 46 is designed to run 20w 50and I can generally stretch a customers hydro life a few more seasons by draining the 10W 40 & filling with 20W 50.
I have even tried 40W 70 ( harley oil ) with mixed success .

Now being an "I can fix anything " type of guy, when I did not know any better I ripped the trans apart pulled out the pump/motor unit, machined the wear surfaces dead smooth ( cause I don't have the equipment to measure surface roughness ) , then paked out the entire units to take up the amount of machining.

It was a drastic failure so down to TT to get the $ 800 drop in service unit .

Some years latter a hydro machine shop opened and he showed me how to do it properly ( although he did not tell me the roughness index ) .
So there you have it
Too rough & it won't work & too smooth & it won't work either .

Also what 99.9999999999999999999999999999999999999999999999999999999999999 % of the face book oil experts can not understand is the oil is thickest when cold and gets thinner when it gets hot
And this applies to multigrades just the same as it does to mono grades except multigrades get thinner slower than mongrads and will tend to plateu .
Thus a 10W oil can never be more viscious than the 10 W it was from the start
So a 20W 50 will be thicker than a 10 W 50 from cold to about 60 - 75 Deg C
The second number in a multi grade is the viscosity at 100 C and it will always be thinner than the base oil the additives were added to
Oils are not like gravey, they do not get thicker as they get hot.
 

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One thing that slips my mind is that my local farm stores carry Mystic oils in heavy weights for really cheap. I figure my next hydro oil changes in my Cubs will be Mystic syn-blend 15W-50 from Atwood's or Orshelin's. I can get it for $28 for 2 gallons, $3.50/qt.

I run Mobil one 15w-50 synthetic in my zero turns hydros.
I can get it near anywhere -Wal Mart, nearly all local auto parts stores, Etc.
 

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Hmm, maybe @TUDOR can chime in here since he seems to know his stuff about hydraulics, but IMO that should work alright.
Operating temperatures for hydros range from ambient at startup to 192° as the maximum under high ambient temperature at heavy loads. Normal operating temperatures range from 120 - 180°. The viscosity of the oil is an indicator of the range of ambient temperature startups. This is the critical information needed by consumers and the tractor manufacturers often post charts in the operator's manual recommending the proper viscosity for various ambient temperature ranges. It will have no effect for practical purposes at normal operating temperatures.

In race car engines and motorcycle engines where oil temperatures routinely rise above 200°, a 20-W-50 oil has benefits, but neither of those machines push the fluid to the same high pressures found in a hydro. When a hydro reaches that kind of temperature, its performance begins to fall off.

While some have adapted their zero turns for snow duty, the vast majority are for warm ambient temperature use only with the result that a multigrade 20-W-50 motor oil works fine in the hydro. The advantage of using a synthetic oil is that it handles heat better which results in a slightly cooler running hydro.

Hydraulic pumps, including hydros, can make use of a variety of fluids and viscosities, mostly determined by the operating conditions, cost, and availability. The manufactures may make recommendation for which to use based on testing, and dictate which to use to maintain the warranty, but the end user can use whatever suits the purpose. In some environmentally sensitive areas, even vegetable oil is being used.

Since the OP limits his ZT use to the (relative) cool of the early morning, his "executive decision" should have no repercussions.The use of synthetic fluid should compensate for the lower high viscosity number.

The mechanical side of this discussion.

The hydro growl, or more correctly whine, is courtesy of the pulse frequencies of a positive displacement pump as each chamber of the pump unloads into the system. As the pressure rises, the whine gets louder. Anyone who has worked in a hydraulic pump room knows this in their bones. Hearing protection is a very desirable piece of equipment.

Cylinder blocks and valve plates are polished to create as smooth a surface as possible in order to limit the space for high pressure fluid to leak out, and to make what space there is as uniform as possible. Fluid does leak out, but at a designed rate for the purpose of lubricating those parts. In this day and age, we have high pressure water jets that cut through steel. While the pressures in a hydro are nowhere near as high, the same effect happens over time with the cylinder block and the valve plate if there are imperfections. Hydros have a much greater service life than a high pressure water jet.

Things that will cause excessive wear in a hydro:

  • Dirt in the fluid
  • High temperatures (Above 225° oil begins to carbonize which produces carbon particles that make a lovely grinding medium. This has caused the deaths of many K46 hydros.)
  • Excessively high pressures. (Pumps are designed for a specific continuous duty pressure. Exceed that pressure for enough time and the cutting action of the fluid under pressure comes into play. This goes hand in hand with the second item and its effect on the K46.)
 

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Discussion Starter #20
NW Arkansas. How about you?
Central Texas. Tractor Supply has most big towns. Behind the friendly clerks it's just another huge evil corporation. But their lubricant prices are good. We patronize the small family ops for feed, hay, etc. As for cattle protein, TSC is half the price. How they manage that, I don't know.
 
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