The 68 is the thicker of the two: at 40 °C it resists flowing almost 50% more than the 46, and that is why it can work hotter. In an ordinary mineral oil (made from refined petroleum, the most common type in hydraulics), the 68 has to be about 8 to 10 °C hotter to become as thin as the 46. The right grade is the one the manual asks for; when it accepts both, the oil temperature at the pump (the part that pushes it through the system) decides.
What is the viscosity of hydraulic oil 46 and 68?
The numbers 46 and 68 measure how thick the oil is, taken at 40 °C: the higher the number, the thicker the oil.
Think of honey and water: honey runs slowly because it resists flowing more. That resistance is called viscosity, and it is what the number on a hydraulic oil measures.
The unit of that measure is the centistoke (cSt). An oil sold as 46 has close to 46 cSt at that temperature, and a 68 has close to 68 cSt. That number is called the ISO VG grade, and it comes from the international standard ISO 3448.
In hydraulic oil, the most common grades run from 10 to 100 (10, 15, 22, 32, 46, 68 and 100), and each one is about 50% thicker than the one before it.
And the letters next to the number, as in HLP 68? They give the oil's class, that is, the protections it carries; they are explained in the article on types of hydraulic oils.
Within the same product line, the 46 and the 68 belong to the same class. The difference in thickness comes from the base oil, the oil that makes up almost all of the lubricant: the 68 has more thick base oil in the blend.
Figure 1. Two identical tubes at 40 °C: the 68 takes almost 50% longer to drain.
For those who need the numbers: under ISO 3448, ISO VG 46 runs from 41.4 to 50.6 cSt at 40 °C, and ISO VG 68 from 61.2 to 74.8 cSt. That is a tolerance of 10% on each side (1 cSt equals 1 mm²/s). The ranges do not touch, and two real oils, each within its own range, can be about 20% to about 80% apart.
The ISO 3448 scale runs from ISO VG 2 to 1500; ASTM D2422 adds 2200 and 3200.
But the machine does not work at 40 °C. What happens when the oil heats up?
How do hydraulic oil 46 and 68 change with temperature?
Both get thinner when they heat up and thicker when they cool down, and the 68 stays thicker than the 46 at any temperature.
Honey does the same: straight from the fridge, it hardly flows; warmed up, it runs fast. Within the same product line, the 46 and the 68 thin out together, like two curves running side by side without crossing.
In practice, the difference between the grades becomes a difference in temperature. The 68 at about 48 °C flows like the 46 at 40 °C, a gap of about 8 °C. With hotter oil, between about 50 and 80 °C, the gap grows to about 9 to 10 °C.
Figure 2. At 48 °C, the 68 flows like the 46 at 40 °C.
How much an oil thins with heat depends on the viscosity index (VI): the higher the index, the less the thickness changes. Ordinary mineral oil sits near 100, and the numbers in this article hold for it.
For those who need the numbers: values calculated with ASTM D341, using VI 100 (ASTM D2270) and oil at the center of the grade; at the edges of the range, the temperatures shift by 2 to 3 °C. In the cold, the gap shrinks to about 6 °C.
| Oil temperature (°C) | ISO VG 46 (cSt) | ISO VG 68 (cSt) |
|---|---|---|
| 20 | 135 | 214 |
| 40 | 46 | 68 |
| 60 | 21 | 29 |
| 80 | 11 | 15 |
If both thin out with heat, what is left is to find out which thickness the machine wants.
Why does temperature decide between the 46 and the 68?
Because the pump only performs well and lasts when the oil stays within a range of thickness, and temperature is what puts the oil inside or outside that range.
The pump manufacturer gives that range in its catalog, and it is called the optimal viscosity range. It applies with the machine at steady state: producing normally, with the oil temperature already stable.
Outside the range, the problem changes depending on the side:
Too thin (oil too hot for the grade): the oil escapes through the clearances, the tiny gaps between parts, and goes back to the tank without moving the cylinder, the part that moves the load. That is internal leakage. The machine slows down, and the oil film, the layer that separates the parts, gets thinner.
Too thick (cold oil, at startup): cavitation, in three steps.
The oil reaches the pump slowly and the pressure drops at the inlet.
With low pressure, the lighter parts of the oil turn to vapor, even with the oil cold, and form bubbles.
Where the pressure rises again, inside the pump, the vapor bubbles collapse all at once and tear particles out of the metal: that is cavitation. The air bubbles that come out of the oil in the same pressure drop do not collapse like that: they cause foam and noise.
How this shows up in your plant: with oil too thin, the cylinders slow down toward the end of the shift, the pressure fluctuates and the tank runs hotter than normal. The reason is that the leakage through the clearances turns into heat and thins the oil even more.
With oil too thick, the pump whines or sounds like gravel in the first minutes and the motor draws more current at startup.
In Figure 3, the pump's optimal range appears in color; the temperatures where the curve of each grade enters and leaves it are the good ones. The horizontal line at the height of the 46 shows the 8 °C difference between the grades.
The vertical axis uses a logarithmic scale: the distance between 10 and 100 cSt is the same as between 100 and 1,000, which lets very different values fit on the same chart. The dashed red line marks the minimum of 10 cSt, accepted only for a short time.
Figure 3. With the optimal range from a pump catalog, the 46 works between about 45 and 67 °C, and the 68 between about 54 and 77 °C. At the same thickness, the 68 is about 8 °C above the 46.
For those who need the numbers: piston pump catalogs usually ask for 16 to 36 cSt in continuous duty and accept 10 cSt only for a short time. On a cold start, they accept up to 1,000 or 1,600 cSt, depending on the catalog, for a few minutes and without load.
With VI 100, 1,000 cSt is reached near minus 6 °C for the 46 and near 0 °C for the 68. Vane and gear pumps have their own ranges.
Some pumps have a case drain (the case is the metal housing around the internal parts): the line that returns to the tank the oil that leaked inside the pump. That oil comes out hotter than the oil in the tank, and for these pumps the catalog usually gives the range for it.
With the pump's range and the curve of each grade in hand, the choice becomes a table lookup.
Hydraulic oil 46 or 68: which one should you use in your machine?
Use the grade the machine manual asks for; if it accepts both, choose by the oil temperature at the pump at steady state.
Look for the grade in the manual. If it gives only one, follow it.
If it accepts both, measure the oil temperature at steady state at the point the pump catalog indicates (the tank or, on pumps that have a case drain, the drain).
If the catalog talks about the drain and you measured in the tank, the tank reading is lower than the drain's: add a few degrees to it. The Technical Team can confirm how many for your pump.
Find the row in the table and check it against the pump catalog.
| If the oil at the pump, at steady state... | Use | Why |
|---|---|---|
| Stays below 45 °C | ISO VG 32 or 46 from the Supra AW line, according to the catalog | The 46 is thicker than the optimal range and only works if the catalog accepts it; the 32 is the grade just below |
| Stays between 45 and 54 °C | ISO VG 46 | The 46 is in the optimal range; the 68 would still be too thick for it |
| Stays between 54 and 67 °C | 46 or 68, according to the catalog | Both stay in the range. The 46 starts more easily in the cold; the 68 leaves margin for summer |
| Stays between 67 and 77 °C | ISO VG 68 from the Supra AW line | The 46 is already too thin. Hotter oil ages faster: it is worth looking at cooling |
| Goes over 77 °C | Investigate the heat source before changing the grade | Even the 68 gets too thin for the optimal range (see the last frequently asked question) |
| Starts cold and works hot (machine outdoors, winter startup) | High viscosity index oil, the HVLP class, in grade 46 or 68; Kelpen Oil formulates the Supra HP line, which meets DIN 51524 part 3 | It thins less when hot and thickens less when cold |
The temperatures in the table hold for oil with a viscosity index near 100. In an oil with a higher index, such as HVLP, the distance between the two grades gets larger.
HVLP gets its high index from an additive that holds the viscosity when hot or from a base oil that already has a high index. Its curve is checked on the oil's data sheet or in an analysis, because the pump catalog gives only the range.
With water in the oil or a fire risk, the choice becomes one of oil type.
Water gets in through condensation, washdown or a worn seal. Where a little gets in all the time, Kelpen Oil formulates the Supra AWD line, of the HLPD class: the D, for detergent, means the oil keeps the water in fine, scattered droplets and carries the dirt to the filter. It does not separate the water in the tank, and water ingress is still dealt with at its source.
Where a leak can meet fire, Supra HFDU FR 68 is harder to ignite than mineral oil, although it remains combustible. It is an ester base (a synthetic oil, made by chemical reaction and not refined from petroleum), with no mineral oil and no water.
For those who need the numbers: the table uses the range of 16 to 36 cSt. By the Arrhenius rule of thumb, only approximate, in mineral oil above about 60 °C, every 10 °C more roughly halves the oil's life against oxidation. The full line is at industrial hydraulic oils.
And when the machine already has 46 and the stock only has 68?
Can you use hydraulic oil 68 instead of 46?
You can, if at steady state the 68 stays within the pump's range and the cold start stays within the catalog limit. The 46 in place of the 68 also works, if the machine runs cool enough for it not to get too thin.
With the 68 in place of the 46: it leaks less through the clearances, but starting gets harder and, with cold oil, the system loses more energy pushing the oil through the lines and valves.
With the 46 in place of the 68: starting gets easier, but the hot oil may get too thin and the cylinder loses speed.
Beware of a tempting swap: putting in 68 because the machine got slow. That can hide a worn pump or cylinder seal. With thicker oil, the worn clearance leaks less and the symptom eases for a while, but the wear goes on, and a little more of it already cancels the gain.
A slow machine also comes from oil that is too hot or from a relief valve (the one that limits the maximum pressure) stuck open all the time. Change the grade only with a measured reason and respecting the manual.
Figure 4. In a worn clearance, the 68 leaks less than the 46, but still more than a new clearance: it hides the wear without fixing it.
For those who need the numbers: in the clearance, the oil flows in layers that slide without mixing (laminar flow). In that regime, the leakage is inversely proportional to the viscosity and grows sharply with the clearance: a clearance 10% larger, leakage about 33% larger (the cube of the clearance).
At 40 °C, the 68, almost 50% thicker, leaks about a third less than the 46. But a clearance only about 14% larger brings the leakage back to what it was (cube root of 68/46 = 1.14).
Another question comes up when topping off the level: can you put the two together?
Can you mix hydraulic oil 46 and 68?
You can, if both are from the same line and the same class. The mixture becomes an oil of intermediate thickness and stays a 46 or a 68 only when it has a little of the other (percentages with both oils at the center of the grade):
with up to about 25% of 68 in the 46, it is still a 46;
from about 25% to 74% of 68, the mixture sits between the two and belongs to no grade;
with more than about 74% of 68 (less than 26% of 46), it is a 68.
When topping off, use the same grade and the same line.
Mixing different classes, such as an HLP with the detergent HLPD, can worsen water separation, foaming and filtration. Additives of different chemistry can react with each other and form sludge, a paste that clogs the filter. Even between two HLP oils from different brands, talk to the Technical Team first.
Figure 5. With both oils at the center of the grade, up to about 25% of 68 the mixture is still a 46; above about 74% of 68 it is a 68; in the middle it is neither.
For those who need the numbers: under ASTM D7152, with both at the center of the grade, a half-and-half mix gives about 56 cSt at 40 °C, outside any grade.
Mixing, heat and contamination change the oil's thickness, and that is why it gets measured.
How does Kelpen Oil check that an oil is really a 46 or a 68?
By measuring the viscosity. In the Kelpen Oil in-house laboratory in Arujá (SP), the viscosity of every batch is measured at 40 °C, and also at 100 °C when the viscosity index is calculated. A result outside the range keeps the batch from leaving the plant; the full list is in the tests that approve every batch.
With the two measurements, you can plot the real curve of that oil, as in Figure 3. In oil in use, the same measurement shows whether it has left its grade:
Viscosity rising usually points to aging from heat and oxygen (oxidation), water in the oil or mixing with a thicker oil.
Viscosity falling usually points to contamination with a thinner fluid, mixing with a lower grade or, in HVLP, which depends on an additive to hold viscosity when hot, breakdown of that additive.
For those who need the numbers:
| What we measure | What it reveals | Standard and equipment |
|---|---|---|
| Viscosity at 40 °C | Whether the oil is in the grade's range under ISO 3448 | ASTM D7042, SVM 4001 |
| Viscosity at 100 °C | The second point of the curve | ASTM D7042, SVM 4001 |
| Viscosity index | How much the oil thins with heat | NBR 14358, SVM 4001 |
Frequently asked questions
Hydraulic oil ISO VG 32 or 46? The 32 is the grade below the 46, for oil that works cooler or a machine that starts cold. The choice follows the manual and the oil temperature at the pump (the 32 is the first row of the table). Under ISO 3448, the 32 runs from 28.8 to 35.2 cSt at 40 °C.
Is there such a thing as SAE 68 hydraulic oil? Not with that number: 68 is an ISO VG grade. SAE is the automotive oil scale (SAE J300 for engines; SAE J306 for gear and axle oils), measured at other temperatures. Some mobile machines use oil sold by SAE grade in the hydraulics, but one number does not convert directly into the other: what the manual asks for is what counts.
What does AW mean in hydraulic oil 68? AW comes from anti-wear: an additive forms a film that wears away instead of the metal when the oil film thins under pressure. From ISO VG 10 to 150, the range DIN 51524 part 2 covers, the Supra AW line meets the standard (HLP class). The 220 and 320 of the line are above that range; for them, what the product states counts: talk to the Technical Team.
What is the thickest hydraulic oil? Between the 46 and the 68, the 68. The ISO VG scale goes on above it, but in hydraulics grades above 100 are uncommon, and what the machine manufacturer asks for counts. The Supra AW line goes up to ISO VG 320, with thicker grades for bearings (the supports in which shafts turn) and hydraulic systems.
Hydraulic oil running hot, what could it be? Frequent causes: a dirty heat exchanger (the oil radiator), a low tank level, a relief valve stuck open all the time and a worn pump, whose internal leakage turns into heat. Thickening the oil treats the symptom; the first step is to find the heat source.
Glossary
Viscosity: the oil's resistance to flowing; it drops when the oil heats up.
ISO VG: viscosity grade from ISO 3448, equal to the middle of the range allowed at 40 °C, in cSt.
Optimal range: viscosity range in which the pump performs well and lasts, given in its catalog.
Steady state: machine producing normally, with the oil temperature already stable.
Case drain: on pumps that have one, the line that returns to the tank the oil that leaked inside the pump.
Internal leakage: oil that escapes through the clearances and returns to the tank without moving the cylinder.
Oil film: layer of oil that separates moving parts.
Talk to the Technical Team
The Kelpen Oil Technical Team looks at the oil temperature at steady state, the pump catalog, the startup temperature and the machine's history and confirms the right grade. If the oil is already in use, the Arujá (SP) laboratory does the analysis of oil in use, as a paid service. Before changing the grade, measure the oil temperature at steady state and write down what the manual asks for.
