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I'm confused about something and maybe someone could clarify. The Hydro-Gear trans uses hydraulic fluid yet the manual states to use 20-40 motor oil. Having two cars that have used hydraulic fluid, I know there is a distinct difference between the two. Further, Gravely sells a synthetic hydraulic fluid in 15w50 and 20w50. Any thoughts????

RustyBoy said:

I'm confused about something and maybe someone could clarify. The Hydro-Gear trans uses hydraulic fluid yet the manual states to use 20-40 motor oil. Having two cars that have used hydraulic fluid, I know there is a distinct difference between the two. Further, Gravely sells a synthetic hydraulic fluid in 15w50 and 20w50. Any thoughts????

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Yes... follow the manual My Gravely manual says to use Mobil1 15w50 oil. Hydraulic fluid isn't necessarily what you think it is. It's a generic term.
Hydraulic fluid - Wikipedia, the free encyclopedia

Like Mike said.... Is not always going to be what you may expect. Hydraulic fluid in some systems can be water.
Depends on what the system is designed to do & utilize.
A good multi-viscosity synthetic Motor Oil is the best "fluid" to run in almost every hydraulic mower. :smile:KennyV

Mike88se said:

Yes... follow the manual My Gravely manual says to use Mobil1 15w50 oil. Hydraulic fluid isn't necessarily what you think it is. It's a generic term.
Hydraulic fluid - Wikipedia, the free encyclopedia

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Ok, if I follow my manual, which I will, states "Use 20W-50 engine oil with an SL API classification." Unlike your manual, my manual isn't brand specific for the oil nor does it mention synthetic. Lastly, even more confusing, there is a "Warning" for hydraulic fluid while there isn't a warning for motor oil, so the manual in this regard is making a distinction between hydraulic fluid and motor oil. However, I prefer synthetic versus conventional oil.

As said, use the 20W-50 motor oil. The SL rating will be on the oil container listing it as so. As far as brand of oil-ANY 20W-50 rated by API as SL will be acceptable. Don't over think it.

RustyBoy said:

Ok, if I follow my manual, which I will, states "Use 20W-50 engine oil with an SL API classification." Unlike your manual, my manual isn't brand specific for the oil nor does it mention synthetic. Lastly, even more confusing, there is a "Warning" for hydraulic fluid while there isn't a warning for motor oil, so the manual in this regard is making a distinction between hydraulic fluid and motor oil. However, I prefer synthetic versus conventional oil.

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The advice you are getting about following the recommendation in your owner's manual is spot on!!!

My Gravely manual(and I checked the Hydro-Gear website to confirm) states conventional 20W-50 oil. I am not going to get into a debate with any users(no offense intended:0) that believe synthetic oil is the greatest thing since sliced bread but I did go to Advance Auto Parts and got a quart of Pennzoil 20W-50 to keep my reservoirs topped up. And that is what I will use in the trannies whenever the fluid/filter service is performed. When I do my 75 hour fluid/filter change I will probably stock up on Pennzoil 20W-50 to take care of the next several services. You will find that since almost all new cars since the early 2000's are leaning towards the lighter weight 5W-30 oils(for obvious emission standards), this has made the formerly popular heavier weight oils harder to find(I used Castrol 20W-40 in my MG's for years...I owned 4 different ones). I am now an avid Pennzoil user on everything I own and with a 1995 Z71 Chevrolet Silverado with 239,000 miles on the original 350 V8 that still runs like new I can attest to how well it protects engines(I have used Pennzoil 10W30 since I drove that truck off the dealer's lot in 1995). Of course, the oil brand is of little importance, if it a well-known brand, but you need to use the 20W-50 oil as recommended. That's my story and I'm sticking to it:0)

Natureboy said:

As said, use the 20W-50 motor oil. The SL rating will be on the oil container listing it as so. As far as brand of oil-ANY 20W-50 rated by API as SL will be acceptable. Don't over think it.

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Yep, all former and current API rating oils are "compatible" on multi-viscosity engine oils.

The primary recommended lubricant is Gravely / Ariens 00057100 15W50 hydro fluid, which
is "fully synthetic". So a synthetic based product is not necessarily a bad thing.

Interestingly several Ariens manuals do mention that this fluid (prod. # 00057100)
is actually the Mobil-1 15w50 synthetic (probably the "extended performance" since they mention this is a high TBN product.
The TBN refers to the level of alkaline acid neutralizers. The ZDDP content of the Mobile-1 15w50 is 1200ppm.
Could be a bit higher, but not bad.

So it looks like you don't have to pay fifteen bucks a quart for the rebadget Gravely fluid, you can pick up
the mobil-1 at wallymart for around $22 bucks for the 5 quart jug. BTW mobil has a $15 rebate going right now on this oil.

Google brought me here ... as far as I'm aware only Gravely refers to motor oil as hydraulic fluid. Look for 15W50 (in my case that's what the owner's manual specs) and what you find "motor" oil for sale.

My interpretation is that it's a lube when used in an engine, or a power conveyance when used under higher pressure to turn wheels.

They could have added a helpful sentence for clarity since you won't find "15W50 hydraulic oil" for sale unless it's in a red Gravely container that costs twice what Mobil 1 does.

Best analogy I can think of is something like 303 tractor fluid oil. It could be a transmission lube, but it also raises the front blade (for example).

"Honey, the tractor needs more fluid oil."

"Oh, so that's different than solid oil?" LOL

That like buying organic chicken vs non organic.. What is the non organic made out of, rocks or some other non organic material?

It is not entirely clear why they specify a 15w50 or 20w50 motor oil for this application.


Some people think it is just a lazy spec, which goes like "oh well motor oils are easy to get so why not"
There are some who think the multigrade SAE viscosity they ask for is optimal for the high heat, high oil pressure app.

We do not have a detailed explanation of their reasons, but we do not have much of a choice, other than following
the manufacturer's recommendation. The only choice I see is selecting a traditional or synthetic oil, fitting the
known parameters they spec-ed.

I have seen motor oil spec-ed in lawn tractor tranny's too, my Scotts tranny asked for the
same thing, 20w50 motor oil, so this is nothing new.

From what I gather, the requirements are some minimal level HTHS, 15w50 or 20w50,
I think there is an implied high anti-wear and anti-oxidant additive content "ask" here,
as the heavy multigrades specified do not limit the anti-wear additives, neither would the ancient SF spec or whatever
they actually asked for.

I prefer synthetics for stability over a long time. For me that translates to some mostly pao or gtl+pao mix oil.
I talked to M1 support, and they confirm that the M1 15w50 red label synthetic is a recommended product
for this application. So that is one safe bet.

There are some oils that seem to be custom engineered for these hydros, e.g.: amsoil . I imagine the specific requirements
could be obtained by lubricant formulators, so that they can design a product that is optimal. Those 2 ( m1 and Amsoil) would be my top
2 choices for this.

I see no point of paying the unreasonably high prices for the OEM branded "hydro fluids" (Stens, Gravely, Scag, Hustler, Hydrogear etc... they all have their own brand) ,
which are either of undislosed composition (often mineral), or just rebranded synth blend/synthetic motor oils.

Oils are liquids so they do not compress very much
As such they can be used for power transmission as well as lubrication.
And as most pumps generate friction and need both lubrication and a means of removing heat so oil is fine in a hydro transmission.
All oils are basically the same but we put extra stuff in there to do things like stop it from frothing , hold crud in suspension or drop crud to the bottom, take more pressure , prevent burning, prevent boiling, prevent freezing , wet surfaces better, not wet surfaces etc etc etc .

Purely synthetic oils are made by converting gases like propane into oil.
They are very expensive because it takes a lot of gas to make a tiny amount of oil and that gas could have been sold as LPG.
All the others are nothing more than oil broken down into all of its component parts then reassembled using only exactly what you need for the product.
In reality they should be called "Super refined" or "Reassembled " oil, synthetic is not the correct description.
However because of the extral processing needed, they are also more expensive but not as expensive as truely synthetic oil .

Now refining is expensive so what generally happens is they take out what is very dangerous to the end use , what is not needed but can be sold for a profit and what is left over is engine oil.
The important thing is it is cheap.

We do the same thing with a mile of products
Soft serve Ice cream is not ice cream it is made from corn starch so we could be calling it "synthetic ice cream " but because we eat it, the idea of eating "synthetic food " is science fiction creepy , so it is not done .
However oil goes into a machine and a machine is man made so oil that is man made ( synthetic ) must be better , obviously.

Now hydros have been around for ages
The 1956 , 100Hp Oliver draught tractor has a hydro drive , just in case the 2 mph bottom gear is too fast for whatever the impliment you are towing is doing.
This hydro runs off the main hydraulic pump then goes through a second hydro pump then the motor then the bull gear and as such uses hydraulic oil
It could have used the engine oil or even the gear oil but the hydraulic oil is already under high pressure so requires less energy to boost the pressure than either of the other two would.

I'd use the mower or hydro makers branded fluid. These toys can cost over 10 grand. No doubt the manufacturer has formulated their oil with special additives for that application. Also protects your warranty.

Using car oil off the shelf from Walmart makes 0 sense. As in the cheapest so called synthetic (Mobile 1) you can buy.

slomo