5W-30 or 5W-40: how to choose the right viscosity
In short
Use the viscosity printed in your owner's manual: 5W-30 is required by most modern engines with a particulate filter and variable valve timing, while 5W-40 suits older or heavily loaded engines and cars whose manufacturer explicitly allows both. The second number does not mean better protection, only thicker oil at 100 °C, and oil thicker than the engine was designed for slows hydraulic actuators and raises fuel consumption.
What 5W-30 and 5W-40 actually describe
Both markings are SAE viscosity grades — a measure of how the oil flows at two different temperatures. The number before the W (for winter) describes cold behaviour: a 5W oil stays fluid enough for the pump to push it to the bearings within the first seconds after start-up, even below freezing. On this point the two oils are identical — 5W-30 and 5W-40 start equally well in winter because they share the same first number. The difference is the second number, which describes viscosity at 100 °C, meaning at operating temperature. A warm 5W-40 stays thicker than a 5W-30. Related to this is a figure manufacturers rely on but rarely print on the front label: HTHS, the high-temperature high-shear viscosity measured at 150 °C. 5W-40 oils generally have a higher HTHS, while many modern 5W-30 oils are deliberately formulated with a lower one to save fuel. That is why the two are not interchangeable. They are not simply thicker and thinner versions of the same product, but oils built to different design targets.
When 5W-30 is the right choice
5W-30 is the default viscosity for most cars built in the last twenty years. Modern engines have narrow oil galleries, variable-displacement oil pumps and variable valve timing systems that use the oil as the hydraulic fluid controlling the camshafts. All of that is calibrated around a specific viscosity, and thicker oil arrives late where it is needed. The second reason is exhaust aftertreatment. If your car has a particulate filter — DPF on a diesel, GPF on some direct-injection petrol engines — the oil must belong to the ACEA C classes, with reduced ash content. The vast majority of oils carrying approvals such as VW 504.00/507.00, MB 229.51 or dexos are exactly 5W-30. Here the choice is not a preference: the wrong class carries ash into the particulate filter, and ash does not burn off during regeneration — it accumulates permanently. If the manual asks for 5W-30 and a C class, the discussion ends there.
When 5W-40 genuinely makes sense
5W-40 remains the correct choice for a large share of the Moldovan car fleet: petrol and diesel engines from the 2000s without a particulate filter, for which the manufacturer specifies something like ACEA A3/B4 together with approvals such as VW 502.00/505.00 or MB 229.5. These engines were designed around a thicker oil film and larger internal clearances. It also makes sense under high thermal load, provided the manufacturer explicitly permits both viscosities: towing a trailer, mountain roads, long summer motorway runs, heavily loaded turbocharged engines. The turbocharger is the component most vulnerable to oil thinning, because its bearing runs at very high temperatures. Watch out for one common trap. Many manuals include a viscosity chart based on ambient temperature. That chart applies only to oils that also meet the required specification. A 5W-40 in class A3/B4 does not become acceptable in a car with a DPF just because the chart shows a 40 grade for summer temperatures.
The myth of thicker oil for a worn engine
It is the most repeated piece of advice in small workshops: if the engine burns oil or rattles when cold, switch to something thicker. In the short term it appears to work, because a more viscous oil passes less easily by worn valve stem seals and piston rings, so measured consumption drops and the noise softens. The problem is everything else. Thicker oil reaches the bearings and camshafts more slowly in the first seconds after a cold start — precisely when most engine wear happens. On engines with variable valve timing or hydraulic lifters, that delay in oil pressure turns into sluggish actuation and new noises. Meanwhile the real cause — hardened stem seals, rings stuck with deposits, a blocked crankcase ventilation system — stays exactly where it was and keeps getting worse. Thickening the oil hides a symptom. If an engine consumes oil, the right move is to find out why, not to bury the number.
The manufacturer approval beats personal preference
Viscosity is only half the decision. The other half is the specification: ACEA, API and, above all, the carmaker's own approval. Those approvals are proprietary test programmes run on specific engines, and oils that pass them earn the right to print the approval number on the can. The ones seen most often here are VW 502.00, 505.00, 504.00 and 507.00; Mercedes-Benz 229.5, 229.51 and 229.52; BMW Longlife-01 and Longlife-04; GM dexos1 and dexos2; Renault RN17; PSA B71 2290. The correct order is simple. Open the manual at the lubricants section, or look at the oil filler cap, where many cars print the specification directly. Write down the required specification. Only then choose a viscosity, from the ones permitted for the local climate. If the manual asks for 5W-30 ACEA C3 with MB 229.51, a 5W-40 A3/B4 is not a superior alternative — it is simply the wrong oil, however respected the brand. One useful detail on the back label: wording such as meets the requirements of or recommended for is not the same as a licensed approval, which appears plainly as a number in the approvals list.
What this means in practice in Moldova
Our winters drop towards −20 °C in some years, and summers regularly pass 35 °C. A 5W first number comfortably covers local cold, while 0W brings a real advantage below −15 °C and for cars that sleep outdoors. Both grades are engineered to handle summer heat — the second number matters under load, not in the car park. If your car has covered more than 200,000 km and the manual permits both viscosities, 5W-40 is a reasonable choice. If the manual lists only one viscosity, stay with it. And if you cannot find the manual, do not guess from the engine type or from whatever the last workshop poured in: call us on +373 22 009 350 with the VIN and we will check the factory requirement together. The MANNOL catalogue carries both viscosities in versions with the approvals common across the European fleet, priced in MDL and shipped from our warehouse in Chișinău.
Need the part we wrote about?
The catalogue carries the right products, priced in MDL with delivery across Moldova. If you are unsure about fitment, call us and we will check it against your VIN.
Frequently asked questions
Can I mix 5W-30 and 5W-40?
In an emergency, if both oils meet the same specification your manufacturer requires, topping up with a litre will not destroy the engine. The resulting viscosity will sit somewhere between the two, so treat the mixture as temporary and return to the correct oil at the next service.
Is 5W-40 better for hot summers?
Not automatically. The second number describes viscosity at 100 °C, the operating temperature the engine also reaches in winter, not the outside air temperature. 5W-40 can help under heavy load — towing, long trips with a fully loaded car — but only if the manufacturer approves it for your engine.
What does the W stand for?
It stands for winter and marks the oil's cold performance. The lower the number before the W, the more easily the oil flows at low temperatures and the sooner it reaches moving parts after start-up.
My engine burns less oil since switching to 5W-40. Is that good?
It shows that thicker oil passes less easily by worn parts, not that the problem is fixed. The valve stem seals, rings or crankcase ventilation remain in the same condition, so a proper diagnosis is still worth doing before the fault worsens.