The issue of choosing lubricants for an internal combustion engine always causes heated debate among car enthusiasts and mechanics. Recently, the viscosity characteristics have attracted special attention. 5W-90, which differs significantly from the usual standards. Many drivers confuse this parameter with the more common 5W-40, not understanding the fundamental difference in the behavior of the fluid at high temperatures.
Understanding oil labeling is not just theoretical knowledge, but a necessity for extending the life of the power unit. If you see the designation 5W-90 on the canister, this indicates the specific operating conditions for which the product is designed. In this article, we will analyze in detail the physical and chemical properties of this type of lubricant and determine in which cases its use is justified, and in which it can cause irreparable harm.
The modern market is overflowing with offers, and unscrupulous sellers may offer oil with extremely high viscosity under the guise of a universal solution. Let's figure out where the truth lies.
Interpretation of viscosity and physical meaning of marking
To properly understand what a liquid labeled 5W-90 is, you need to refer to the standard SAE J300. This classification determines the viscosity-temperature properties of motor oils. The first number "5" in combination with the letter "W" (Winter) indicates low temperature viscosity. This means that at low temperatures the oil behaves like a class 5W, ensuring engine crankability and lubricant pumping through the channels down to minus 30-35 degrees Celsius.
However, the second number “90” arouses the greatest interest and at the same time bewilderment among specialists. In the usual classification of motor oils for passenger cars, we most often find values of 20, 30, 40 and, less often, 50 or 60. The number 90 in the high temperature range (at 100°C) indicates an extremely thick film. For comparison, standard 5W-40 oil, when heated to operating temperature, has a viscosity corresponding to class 40, while 90 is almost two and a half times thicker.
High kinematic viscosity at 100 degrees Celsius creates a powerful protective film between rubbing parts. This is critical for components with large gaps, which are formed as a result of long-term operation or design features of old engines. However, the use of such a thick liquid in modern engines with narrow oil channels and a complex system of hydraulic compensators can lead to oil starvation.
- 🔍 Low temperature start: Provides reliable engine starting in winter, similar to 5W-30 or 5W-40 class oils.
- 🛡️ Protective film: Forms a very thick layer of lubricant at high temperatures, preventing direct metal contact.
- ⚙️ Pressure stability: Maintains high pressure in the oil line even under high heat and heavy loads.
It is worth noting that oils with an SAE viscosity of 90 are more often associated with transmission fluids. If you come across motor oil with this marking, you are looking at either a product for specific heavy equipment, or oil for air-cooled engines, or the result of mixing base oils with thickeners.
Differences between 5W-90 and popular analogues 5W-40 and 5W-50
Comparing 5W-90 oil with more conventional analogues allows you to clearly see the limits of its application. The main difference lies in the behavior at engine operating temperatures, which are usually between 90 and 110 degrees Celsius. Oil 5W-40 is the “golden mean” for most modern engines, providing a balance between protection and fuel efficiency.
Unlike 5W-40, 5W-90 oil creates significantly more resistance when the pistons move and the crankshaft rotates. This phenomenon is known as friction loss. As a result, the engine may become noisier and fuel consumption will inevitably increase. However, in situations where maximum sealing of piston rings and protection of worn journal bearings is required, high viscosity comes to the fore.
Considering the 5W-50 option, we see a closer “relative” to 5W-90, but even here there is a significant difference. Fiftieth oil is still considered the upper limit for many sports and heavily loaded civilian engines. The ninetieth oil moves into the category of specialized products.
⚠️ Attention: It is strictly not recommended to pour oil with a viscosity of 90 into new engines with a variable valve timing system (VVT-i, VTEC, VANOS). The hydraulic mechanisms of these systems will not be able to quickly respond to changes in operating conditions due to the high inertia of thick oil, which will lead to errors in engine operation.
The table below shows the comparative characteristics of different viscosity grades at high temperatures, which helps to understand the extent of the differences:
| Parameter | 5W-30 | 5W-40 | 5W-50 | 5W-90 (conditional) |
|---|---|---|---|---|
| Kinematic viscosity at 100°C (cSt) | 9.3 - 12.5 | 12.5 - 16.3 | 16.3 - 21.9 | > 20.0 (up to 30+) |
| Temperature range (min) | -30°C | -30°C | -30°C | -30°C |
| Application | New cars | Average mileage | Sports/Old cars | Special equipment/Gearboxes |
| Fuel consumption | Low | Medium | High | Very tall |
Thus, the transition from 40 to 90 is not just a step to the side, but a radical change in the operating mode of the entire power unit. It must be taken into account that the oil pump must have sufficient capacity to push such a thick liquid through all components of the lubrication system.
Areas of application: where 5W-90 oil is really needed
Looking at the characteristics, it may seem that 5W-90 oil is an unnecessary product for the civilian automotive industry. However, it has its own, albeit narrow, niche of application. First of all, such viscosities are in demand in engines old design, released several decades ago, where the technological gaps were initially designed for very thick mineral lubricants.
5W-90 can also be used as a temporary measure for engines with critical wear that are preparing for a major overhaul. If the engine “eats” liters of oil, smokes and loses pressure, filling with a high-viscosity compound can briefly extend its life by sealing the gaps between the rings and cylinders. This allows you to get to the service on your own, albeit with risks.
Another area is specialized equipment operating under conditions of extremely high temperatures or constant overloads, where ordinary oils simply burn out or liquefy to the state of water. Some types of motorcycles with a dry sump or combined lubrication of the engine and gearbox may also require high-viscosity oils.
Can 5W-90 be mixed with 5W-40?
Mixing oils of different viscosities is possible in emergency cases if they have the same base (for example, both are synthetic). However, the resulting viscosity will be intermediate and unpredictable. This mixture is not suitable for continuous use, since the additive package may conflict.
It is important to distinguish between motor oils and transmission oils. Often, manual transmissions use oils with a viscosity of 75W-90 or 80W-90. Mixing up the canisters is extremely dangerous: once in the engine, transmission oil with its specific extreme pressure additives can quickly destroy the catalyst and damage the oxygen sensors.
- 🚜 Old technology: Engines of cars and tractors produced before the 80-90s.
- 🛠️ Pre-sale preparation: Temporary solution to mask engine wear before sale (unethical, but practical).
- 🏎️ Sports engines: Specific assemblies with increased clearances for operation at extreme conditions.
If your vehicle doesn't fall into one of these categories, using 5W-90 oil is likely a mistake. Modern engines are designed with certain hydrodynamic properties of the lubricant in mind, and deviation from them leads to a violation of the thermal regime.
Effect of high viscosity on engine performance and consumption
The use of oil with a viscosity of 90 according to SAE has a direct impact on the economic and dynamic performance of the vehicle. Thick lubricant creates high rotational resistance, which forces the engine to expend additional energy to overcome friction. This phenomenon is especially noticeable during a cold start, when the oil has not yet warmed up, but even at operating temperature the losses will be significant.
Fuel consumption can increase by 5-10%, and even more in the urban cycle with frequent acceleration and braking. The engine becomes more sluggish and the response to the gas pedal slows down. This is due to the fact that hydraulic compensators and phase shifters require more time to fill with oil and adjust the clearances in the mechanisms.
If you do decide to switch to a more viscous oil, be sure to monitor the engine temperature. Thick oil removes heat from rubbing pairs less well, which can lead to local overheating of the piston group.
On the other hand, for a worn motor, high viscosity is a chance to increase the pressure in the system. The oil pressure gauge, which constantly blinks on the instrument panel when using 5W-40, may no longer bother the owner when switching to a thicker composition. But this is only a cosmetic effect: the pressure increases due to flow resistance, and not due to the improvement in the condition of the liners.
The thermal stability of class 90 oils is usually high, and they are less susceptible to evaporation loss. Therefore, oil consumption “waste” may decrease, but this will be compensated by increased fuel consumption. The owner will have to decide for himself what is more profitable: adding a liter of oil every 1000 km or overpaying for gasoline at each gas station.
⚠️ Attention: When using high-viscosity oils in winter, the risk of the bearings turning during a cold start increases. The oil simply does not have time to get into the friction zone a split second before the crankshaft begins to rotate.
Compatibility with materials and filters
Switching to 5W-90 oil requires careful attention to the condition of the filter elements. A standard oil filter may not be able to handle such thick fluid, especially at low temperatures. This can lead to the opening of the filter bypass valve, and then unrefined oil with all the dirt and chips will flow into the engine.
When using high-viscosity oils, it is recommended to shorten filter replacement intervals or select models with a larger filter element area. It is also worth checking the condition of rubber seals and seals. Although modern synthetic bases are safe for rubber, high concentrations of certain additives in specific oils can cause old cuffs to swell or dry out.
Engine materials such as plain bearings (liners) usually tolerate high viscosity because it prevents the oil wedge from being squeezed out. However, aluminum alloy pistons and cylinder heads may suffer from local overheating if heat dissipation is impaired by too thick oil.
☑️ Check before changing viscosity
It is also important to consider compatibility with the catalyst. If the 5W-90 oil is mineral or semi-synthetic with a high ash content, it will quickly damage the environmental system. For vehicles with diesel particulate filters (DPF), such oils are generally contraindicated unless they have the appropriate Low SAPS approval.
Transition rules and replacement intervals
If you have made an informed decision to switch to 5W-90 oil, you need to do it wisely. Simply draining the old oil and adding new oil may not be enough, especially if the engine previously had 5W-30 liquid oil. Residues from the old oil will mix with the new oil, changing its design characteristics.
It is recommended to flush the engine with a special flushing oil or use a “five-minute wait” before replacement in order to remove as much as possible the residues of the previous product. However, you need to be careful with flushing: aggressive chemicals can wash away dirt that will clog the oil channels, and then the thick 5W-90 oil will no longer be able to circulate through the system.
Change intervals often have to be shortened when using high viscosity oils. Despite the fact that the oil itself can retain its properties for a long time, wear and oxidation products accumulate faster in an old engine. The optimal interval is considered to be 5000-7000 km, especially if the operation takes place in urban conditions.
It is necessary to constantly monitor the oil level with a dipstick. Due to the high carbon dioxide resistance, the level can stand still for a long time, creating the illusion of an ideal state, but then drop sharply. Or vice versa, if the engine “eats” oil, the level will drop more slowly, hiding the real consumption.
Switching to 5W-90 oil is a temporary measure to extend the life of an old engine or a solution for specific equipment, but not a way to repair or improve the performance of a modern car.
What happens if you pour 5W-90 into a new engine?
In a new engine, this will lead to increased fuel consumption, noise during operation, possible overheating and the risk of damage to the variable valve timing system due to slow oil pumping.
Is it possible to use 5W-90 in the summer when it’s hot?
Yes, in hot climates and under high loads, high viscosity is justified, but only if the engine structurally allows the use of such oils (for example, it has large clearances).
How to distinguish high-quality 5W-90 oil from a fake?
High-quality oil has clear markings, an even color without inclusions and a specific smell. Counterfeits often have a cloudy sediment, a strong burning smell, or an overly watery consistency. Always check the holograms and QR codes on the canister.
Is 5W-90 suitable for diesel engines?
For old atmospheric diesel engines - perhaps. For modern turbodiesels with particulate filters - absolutely not, unless the oil has the appropriate ACEA C3/C4 approvals and low ash content.
Do I need to flush the engine when switching to 5W-90?
It is advisable to avoid mixing additive packages and changing the final viscosity. However, washing should be gentle so as not to provoke the detachment of large fractions of carbon deposits.