Hydraulic oil 68 transmits force and lubricates the pump in hot or heavy-load machines: presses, injection molding machines, truck cranes and backhoes. The 68 is the viscosity (thickness) grade under ISO 3448: 61.2 to 74.8 cSt (centistokes) at 40 °C, one grade above 46. Picture water and syrup in the same gap: water runs through, syrup holds. Every oil thins out with heat; the thicker 68 leaks less through the pump clearances. Use the 68 when the manual says so.
Which hydraulic oil 68 should you use in each machine?
In most industrial machines, the right choice is an anti-wear hydraulic oil 68 of the HLP class, as defined by DIN 51524 part 2. The cases that depart from this are in the table; the guide to types of hydraulic oil covers the other classes.
| If your plant has… | Use | Why |
|---|---|---|
| A press, plastic injection molding machine or machine tool with hot oil after hours of work | Supra AW 68 (HLP, DIN 51524 part 2) | The anti-wear additive protects the pump under pressure; the 68 grade keeps the oil film in place when hot |
| A truck crane, crane, backhoe or forklift with its own hydraulic tank and a manual calling for 68 | Supra AW 68 | Same pump and cylinder demands, in a mobile machine |
| Outdoor machine, with a cold start in the morning and heat in the afternoon | HVLP 68 (DIN 51524 part 3); Kelpen Oil has the Supra HP line in this class | The HVLP class changes less in thickness between cold and hot |
| Manual calls for zinc-free oil or the HLPD class (HLP with detergent) | Supra AWD 68 (HLPD, DIN 51524 part 2) | A zinc-free line, with detergency that keeps dirt in suspension until it reaches the filter |
| Tractor with transmission, hydraulics and brakes in the same housing | The fluid the manual calls for (usually a universal tractor transmission and hydraulic fluid) | The oil-bath brake needs a friction modifier, and the transmission gears need an extreme-pressure additive; hydraulic oil was not made for either |
| Press or furnace with a risk of oil leaking onto hot metal | Fire-resistant hydraulic fluid, specified by the Technical Team | Mineral oil burns; fire-resistant fluid makes it harder for a flame to spread |
Kelpen Oil's whole line of industrial hydraulic oils is filled on demand. Supra AW 68 comes in a 20 L pail, a 200 L drum, a 1,000 L container (IBC) and in bulk.
What does the 68 in hydraulic oil mean?
The 68 gives the thickness of the oil, measured at 40 °C: it is ISO VG grade 68, which accepts any oil between 61.2 and 74.8 cSt (centistokes, the unit of kinematic viscosity). The higher the number, the thicker the oil: 32 and 46 are thinner; 100 is thicker.
The letters on the label say what the oil contains besides the base oil:
AW 68: AW stands for anti-wear, the additive that forms a protective film on the pump metal when pressure squeezes the oil film. "AW 68 oil" is anti-wear hydraulic oil of grade 68.
HLP 68: the same idea as written in DIN 51524 part 2, the German standard for mineral hydraulic oil with anti-wear, antioxidant and anticorrosion additives. Kelpen Oil's Supra AW meets DIN 51524 part 2.
HVLP 68: part 3 of the same standard; it requires a high viscosity index, that is, an oil that thins out less when it heats up.
An oil labeled only "68", with no letters or standard, gives the grade and nothing else. For a hydraulic system with a pump under pressure, ask for the grade and the class together: "ISO VG 68, HLP".
Why do some machines call for hydraulic oil 68?
Because these are machines in which the oil works hot, and the 68 stays thick enough to seal the pump and keep the parts apart at that temperature. The mechanism is this:
The pump pushes oil through clearances of a few thousandths of a millimeter between gears, vanes or pistons and the housing; the oil itself seals these clearances.
Every oil thins out when it heats up. An ordinary mineral 68 oil has 68 cSt at 40 °C and drops to about 16 cSt near 77 °C.
Oil that is too thin flows back through the clearance to the suction side, the pump inlet (internal leakage), which leads to less flow at the cylinder and more heat, because the lost energy turns into temperature.
With a thin oil film, the pump parts touch, which leads to wear and metal particles in the oil.
The extra heat speeds up oxidation, the reaction of the oil with oxygen in the air, which leads to sludge and varnish on the valves.
How this shows up in your plant: the press or injection molding machine starts the shift normally and slows down after a few hours; the cylinder loses force when the machine is hot; the pump gets noisier; the tank is too hot to keep a hand on.
Figure 1. Clearance between the tooth and the pump housing, enlarged, with both pumps at 70 °C. The clearance is the same; the thickness of the oil changes. The 46 is near 15 cSt and the 68 near 20 cSt: through the same clearance, the thin 46 flows back much more from the pressure outlet to the inlet.
Piston pump catalogs ask for oil between 16 and 36 cSt in the pump's case drain line (the return of the oil that leaked inside it), with the machine already hot. An ordinary mineral 68 oil sits in that range from about 54 to 77 °C; the 46, from about 46 to 67 °C. That is why a machine that runs hot calls for the 68: at its temperature, the 46 would already be too thin. The values apply to oil with a viscosity index of 100 at the center of the grade and vary slightly from oil to oil.
The full comparison between the 46 and the 68, with the temperature curve of both, is in the article on the difference between hydraulic oil 46 and 68.
Is hydraulic oil 68 suitable for tractors and heavy machinery?
It is when the manual calls for ISO VG 68 hydraulic oil in a tank used only for hydraulics, the case of many truck cranes, cranes, backhoes and forklifts. In these machines the oil works at high pressure and heats up on a heavy workday, which is the condition the 68 grade is made for.
The tractor is the exception that misleads the most. In many tractors the transmission, the hydraulics and the oil-bath brake share the same housing (the shared oil reservoir). These call for the universal tractor transmission and hydraulic fluid, known as UTTO, which contains a friction modifier for the brake and clutch and an extreme-pressure additive for the transmission gears. With ordinary hydraulic oil, the brake may chatter and squeal, the power take-off clutch (the shaft that drives implements) may slip and the gears wear out sooner. Before buying, check the manual for the fluid the tractor calls for and whether the hydraulics have a separate tank; even with a separate tank, use the fluid in the manual.
In mobile machines the climate also matters. In cold-winter regions, with an early-morning start and heavy work in the afternoon, the HVLP class varies less in thickness between the two extremes.
Can hydraulic oil 68 handle high temperatures?
It handles them better than the 46, but the grade only solves the thickness; heat still wears the oil out. The practical rule from the lubrication literature, based on the Arrhenius law of chemistry, says that above 60 °C every 10 °C more cuts the life of mineral oil in half. The 68 at 70 °C is still within the range the pump asks for, but it oxidizes about twice as fast as at 60 °C.
If the tank stays above 60 °C all the time, look for the cause before thinking about a thicker oil:
a dirty heat exchanger or a stopped fan;
a low tank level, which gives the oil less time to cool;
a relief valve open for too long, turning pressure into heat;
a worn pump, with high internal leakage.
Moving up to ISO VG 100 only makes sense if the pump manual accepts that grade. Oil that is too thick makes the pump strain at startup and can starve the suction side. When the oil runs hot for long, check when to change hydraulic oil.
When is hydraulic oil 68 not the right choice?
When the manual calls for another grade or another type of fluid. The most common cases:
Manual calls for 32 or 46: the manual's grade rules. The 68 is thicker at startup: an ordinary mineral 68 reaches 1,000 cSt near 0 °C, the startup limit that many pump catalogs accept only for a few minutes. With oil thicker than intended, the pump strains and the machine loses more energy to friction in pipes and valves.
Automatic transmission or a machine that calls for ATF: automatic transmission fluid is close to ISO VG 32 and contains a friction modifier. Hydraulic oil 68 and ATF are not interchangeable; use what the manual calls for.
Power steering: the fluid also comes from the vehicle manual, and in most cars it is ATF or a dedicated steering fluid.
Gear reducer: it usually calls for gear oil with an extreme-pressure (EP) additive, which protects the teeth when the load breaks through the oil film, even when the grade is also 68. Use the oil the reducer manual calls for.
How does Kelpen Oil check hydraulic oil 68?
In its own laboratory in Arujá (SP), hydraulic oil goes through tests that show whether it is in the right grade and whether it still protects the pump. No batch leaves the plant without laboratory approval; the article on the tests that approve every batch shows how this is done. The same tests work for oil already in use in your machine:
| What we measure | What it reveals | Standard |
|---|---|---|
| Viscosity at 40 °C (SVM 4001 viscometer) | Whether the oil is still within 61.2 to 74.8 cSt or was topped up with another grade | ASTM D7042 |
| Zinc and phosphorus by ICP spectrometry (Optima 7000 DV) | How much anti-wear additive is left, and whether zinc and zinc-free oils were mixed | NBR 14786 |
| Water by Karl Fischer | Dissolved water that corrodes the pump and speeds up oxidation | Coulometric Karl Fischer |
| Acid number (TAN) | How much the oil has already oxidized from heat | NBR 14448 |
| Infrared spectrum | Oxidation and contamination, compared with new oil | ASTM E2412 |
| Particle count (PAMAS counter) | Dirt and wear metal that score the pump clearances | ISO 4406 |
| Foam and water separation | Whether the oil still releases air and water without foaming or emulsifying | NBR 14235 and NBR 14172 |
Frequently asked questions
What is the best hydraulic oil 68? The best one is the one that meets the class in the manual (usually HLP under DIN 51524 part 2) in ISO VG 68, with a viscosity report for the batch. Kelpen Oil's Supra AW 68 meets DIN 51524 part 2.
Is hydraulic oil 68 suitable for bearings? It is in bearings lubricated by bath or circulation when the manufacturer calls for ISO VG 68 anti-wear or circulating oil and does not require an extreme-pressure additive. The Supra AW line was made for bearings and hydraulic systems.
Is hydraulic oil 68 the same as 68 gear oil? No. Both have the same thickness, but gear oil contains an extreme-pressure additive for tooth contact; the article on reducer oil with EP additive explains the difference.
Can you top up with hydraulic oil 68 from a different brand? Between two HLP 68 oils the mix usually works, but zinc oil mixed with zinc-free oil can form deposits and clog the filter sooner. If the mix has already happened, an analysis of the oil in use (zinc, foam and particle count) shows the problem before it reaches the pump.
Glossary
ISO VG: viscosity grade of industrial oil under ISO 3448, measured at 40 °C, in cSt.
cSt (centistokes): unit of kinematic viscosity; the higher it is, the thicker the oil.
HLP: mineral hydraulic oil with an anti-wear additive, under DIN 51524 part 2.
HVLP: hydraulic oil under DIN 51524 part 3, which thins out less with heat.
Internal leakage: oil that flows back through the pump clearance from the pressure side to the suction side without doing work.
Oxidation: reaction of the oil with oxygen, sped up by heat, that forms acid, sludge and varnish.
HLPD: HLP oil with a detergent additive, which keeps dirt in suspension until the filter instead of letting it settle on the valves.
Extreme pressure (EP): additive that protects the metal when the load breaks through the oil film, typical of gear oil.
UTTO: a single fluid for the transmission, hydraulics and oil-bath brakes of tractors.
Talk to the Technical Team
The Kelpen Oil Technical Team evaluates the pump type, the tank temperature with the machine hot, the cold start and the class the manual calls for, and confirms the right product. If the oil is already in use, the Arujá (SP) laboratory performs the analysis of oil in use, as a paid service. Save this article or send it to whoever looks after the presses, injection molding machines and mobile machines in your company.
