By AnnaA machine comes back from a job with slow, sluggish lift response, and the workshop has two pails on the shelf: one labelled ISO VG 32 and one labelled ISO VG 46. Picking the wrong one is not a small mistake, because the grade is tied to how the oil behaves at a defined temperature, not to how cold the morning felt.
The hydraulic oil 32 vs 46 comparison starts with one fact: both are viscosity classes measured at 40 °C. A higher number means a thicker oil at that reference temperature, but the grade alone does not approve a fluid for a machine. The equipment manual's specified fluid, approval, additive package, and stated operating range decide what goes in.
This article is written for remote-controlled equipment after-sales staff and fleet maintenance planners who have to answer that question at the bench. It explains what the grade means, why temperature changes the picture, and where the manual has to take over.
Hydraulic oil 32 vs 46: Explain ISO VG as a viscosity class at 40 °C
ISO VG stands for International Organization for Standardization Viscosity Grade. The number is a class label tied to kinematic viscosity at a reference temperature of 40 °C. When a supplier writes "ISO VG 32" or "ISO VG 46," the reader should understand that the fluid falls into a defined band around that reference point, not that the oil has one fixed thickness at every temperature the machine will see.
Hydroscand's guide on hydraulic oil 32 versus 46 treats the two as neighbouring viscosity classes and explains the choice in terms of the application and the conditions the system runs in. That framing is useful for a workshop: the grade is a starting point for comparison, not a complete instruction. Two oils carrying the same grade can still differ in base stock, additives, pour point, air-release behaviour, and approvals.
For a fleet, the practical consequence is that "we use 46" is an incomplete maintenance rule. It says which viscosity class the team has standardised on. It does not say which product, which approval, or which machine the class is valid for.
Separate grade from a complete fluid specification
A viscosity grade is one line in a much longer fluid specification. A full product data sheet normally also states the ISO viscosity grade, the measured kinematic viscosity at 40 °C and often at 100 °C, the viscosity index, density, flash point, pour point, and performance classifications such as anti-wear (AW) or detergent-type designations. Those details matter because a hydraulic system depends on more than thickness: pump protection, filter compatibility, seal material, oxidation stability, water separation, and air release all sit alongside viscosity.
That is why a grade label on a pail is not the same as an equipment approval. A fluid can be labelled ISO VG 46 and still be the wrong fluid for a given pump if the manual calls for a specific product family, a specific approval, or a specific additive package. Conversely, two products in the same grade may both be acceptable if the manual's requirement is written at the grade and performance level rather than at brand level.
For the after-sales reader, the safe habit is to record three things separately: the grade, the product identity, and the manual reference that authorises it. When a machine arrives at the workshop with no record, the grade alone does not tell you what was in it before.
Use the named Mobil AW data for a bounded comparison
The Mobil Hydraulic AW series product data sheet gives a concrete example of how grade names relate to measured values. In the named data sheet, the ISO VG 32 product is listed with a kinematic viscosity of 32 mm²/s at 40 °C, and the ISO VG 46 product with 44 mm²/s at 40 °C. Both figures are in square millimetres per second, the usual unit for kinematic viscosity.
That comparison is useful for two reasons. First, it shows the direction of the difference: the 46 product is thicker at the reference temperature than the 32 product. Second, it shows that the grade name is a class label, not a measurement. The 46 product's measured value of 44 mm²/s sits inside the band for that grade rather than exactly on the number 46. A workshop that assumes "46 means exactly 46" will misread data sheets.
The comparison is bounded. It describes two named products from one manufacturer's published data sheet. It does not establish that every ISO VG 32 oil measures 32 or that every ISO VG 46 oil measures 44, and it says nothing about which grade a particular remote-controlled machine should use. Treat it as an illustration of how to read a data sheet, not as a recommendation.
| Item to compare | ISO VG 32 example (Mobil Hydraulic AW, named data sheet) | ISO VG 46 example (Mobil Hydraulic AW, named data sheet) | What it does not tell you |
|---|---|---|---|
| Grade label | ISO VG 32 | ISO VG 46 | Whether the fluid is approved for a given machine |
| Kinematic viscosity at 40 °C | 32 mm²/s | 44 mm²/s | Behaviour at other temperatures |
| Relative thickness at 40 °C | Lower | Higher | Pump protection, filter behaviour, seal compatibility |
| Additive/performance family | AW series (per named data sheet) | AW series (per named data sheet) | Suitability for a specific manual requirement |
| Machine approval | Not stated for any SM model | Not stated for any SM model | Anything about Super Mowers equipment |
Explain why viscosity changes with operating temperature
Viscosity is not constant. As hydraulic oil warms, it flows more easily; as it cools, it resists flow more strongly. That is why a grade defined at 40 °C cannot by itself describe what happens during a cold start in winter or after an hour of hard work in summer. The same oil can be too thick to move freely at start-up and thinner than intended once the system is at working temperature.
For a maintenance planner, the question is therefore not only "32 or 46?" but "what temperature range does this machine actually see, and what does its manual say about that range?" A machine stored in an unheated shed and started on a frosty morning is in a different situation from one that works continuously in warm conditions. The grade that suits one duty cycle may not suit the other, and the manual is the document that defines the acceptable window.
Use the named Hydroscand explanation
Hydroscand's comparison of hydraulic oil 32 and 46 frames the choice around the operating conditions and the application rather than around a single universal answer. That is the right shape for a workshop discussion: the grade is selected against the system and its environment, and the supplier's guidance is a starting reference that still has to be matched to the machine documentation.
The same logic applies to the Sinopec Online comparison of ISO VG 32 and ISO VG 46, which discusses the two grades as alternatives with different behaviour rather than as interchangeable pails. Neither source should be read as an approval for a specific Super Mowers model. They are general explanations of how the grades differ.
A practical way to use this material is to build a short temperature profile for each machine or fleet group: the lowest temperature at which it is expected to start, the typical working temperature, and the highest sustained temperature. That profile then becomes the question you put to the manual or the equipment supplier, rather than a guess based on the season.
Do not prescribe a universal oil from climate alone
A common shortcut is to say "cold climate means 32, warm climate means 46." That shortcut is not reliable. Climate describes the air; the hydraulic system sees its own temperatures, which depend on duty cycle, load, cooling, reservoir size, and how long the machine runs. A machine that starts cold but works hard for hours can see oil temperatures well above ambient, and a machine that works lightly in a warm climate may never reach the temperature its grade was chosen for.
There is also the question of what the manufacturer has already validated. If a manual specifies a grade and a performance level for a given model, the correct action is to follow that specification, not to substitute a different grade because the local climate seems to call for it. If the manual gives a temperature range and permits more than one grade within it, the choice should be made inside that permitted range and recorded.
Application review
Discuss your mowing application.
Share the working condition, target market, product family and project scope.
For fleet standardisation, the temptation is to pick one grade for every machine to simplify purchasing. That can be reasonable only after checking that every machine in the group permits that grade across its operating range. Where machines differ, a single fleet grade becomes an undocumented assumption.
| Decision input | Why it matters | Where to confirm it |
|---|---|---|
| Machine model and serial/revision | Different models and revisions can carry different requirements | Machine manual and nameplate |
| Specified fluid and approval | Grade alone is not an approval | Manual's lubrication section |
| Permitted ambient/operating range | Defines whether a grade is acceptable in the intended conditions | Manual's operating limits |
| Actual duty cycle and oil temperature | Determines whether the oil stays in its intended viscosity window | Machine records and, where available, temperature monitoring |
| Previous fluid identity | Mixing or topping up with an unknown fluid creates uncertainty | Service records and handover notes |
| Storage and start-up conditions | Cold starts are a different case from continuous warm running | Site information and manual guidance |
Read the actual equipment and pump requirements
The manual is the controlling document. Before any oil goes into a reservoir, the after-sales team should locate the lubrication or hydraulic fluid section for that exact model and revision, and read what it requires: the grade or grades permitted, any named product or approval, the ambient or oil temperature range, and any note about mixing or flushing. If the manual is missing or the section is unclear, the correct action is to escalate to the equipment supplier rather than to choose by habit.
A real example of how specific these requirements can be comes from a different brand. The McConnel RoboCut operator and parts manual (Rev. 09.10.13) specifies PANOLIN HLP SYNTH 46 for that machine. That is a named product at a named grade in a named manual for a different manufacturer's remote-controlled machine. It illustrates that manuals can be product-specific rather than grade-generic. It says nothing about Super Mowers equipment and must not be transferred to an SM model.
Super Mowers' public page for the Remote 50 gasoline mower mentions an optional hydraulic lift. That is a configuration note on a public page, not a statement that every SM machine shares the same hydraulic circuit, the same pump, or the same permitted fluid. Any fluid decision for an SM machine has to come from that machine's own documentation.
Check approval, additives and operating range
Three checks should happen before a top-up or change:
- Approval. Does the manual name a product, a performance specification, or an approval that the fluid must meet? A grade label is not an approval.
- Additives and fluid type. Does the system require a specific fluid family, such as a mineral anti-wear hydraulic oil or a synthetic product? Mixing families without guidance can change performance.
- Operating range. Does the manual state a temperature range within which the chosen grade is acceptable? If the machine will be used outside that range, the manual or supplier needs to be consulted.
These checks also belong in the service record. A record that says only "topped up with 46" is weak evidence for the next technician. A record that says which product, which grade, which quantity, and which manual reference authorised it is far more useful at the next service or at resale.
Escalate incompatible or undocumented mixtures
If a machine arrives with an unknown fluid, or if two different products have been mixed without documentation, the workshop should not simply add a preferred grade on top. The right step is to escalate: ask the equipment supplier or the fluid supplier what is known about compatibility, and follow the manual's guidance on flushing or replacement if it provides one. This article does not prescribe a flushing procedure, because the correct procedure depends on the machine and its manual.
The same applies to substituting an engine oil for a hydraulic fluid. Engine oils and hydraulic oils are formulated for different duties, and a specification written for one does not authorise the other. The site's existing articles on SAE 30 vs 10W-30 lawn mower oil and on commercial lawn mower oil change intervals deal with engine lubrication; they are not a substitute for a hydraulic fluid specification.
FAQ
Does a larger ISO VG number describe a thicker oil at the reference temperature?
Yes. At the 40 °C reference temperature, a higher ISO VG number describes a thicker oil than a lower one. In the named Mobil Hydraulic AW data sheet, the ISO VG 46 product is listed at 44 mm²/s at 40 °C, compared with 32 mm²/s for the ISO VG 32 product. The grade is a class, so individual products can sit at different points within their band.
Does winter weather alone determine the correct grade?
No. Cold ambient conditions are one input, but the machine's manual defines the permitted fluid and operating range. Duty cycle, load, cooling, and start-up conditions also affect the oil temperature the system actually sees. Choosing a grade from the season alone bypasses the document that authorises the fluid.
Can engine oil specifications be substituted for hydraulic fluid requirements?
No. Engine oil specifications are written for engine lubrication and do not authorise use in a hydraulic system. The hydraulic fluid requirement has to come from the machine's own manual or from the equipment supplier.
Conclusion
ISO VG 32 and 46 are viscosity classes at 40 °C, not machine approvals. Read the data sheet for measured values, then follow the exact model's manual for the permitted fluid and operating range.
Video: ISO VG 32 vs 46 vs 68: Which Hydraulic Oil Should You Use? #blincehydraulic #hydraulicsystem
