The numbers on the bottle (5W-30, 5W-40...): what they actually mean

The code isn't marketing — it's the most important spec on the bottle. The first number, before the "W" (for winter), shows how the oil flows when cold: the lower it is, the faster it reaches every part of the engine right after start-up. The second number measures viscosity with the engine hot, at 100°C: the higher it is, the more "body" it keeps under load and temperature.

A 5W-30 protects just as well when hot as a 5W-40 in most modern engines, and flows a bit better when cold — but if your handbook calls for 5W-40 by design (engine clearances, turbo, high mileage), fitting 5W-30 "because it's basically the same" isn't an upgrade, it's stepping outside the manufacturer's spec.

Synthetic, semi-synthetic or mineral: the real difference

  • Mineral: refined straight from crude oil, with minimal processing. Barely used in modern cars any more — only in very old or low-performance engines.
  • Semi-synthetic: a blend of mineral and synthetic. Good value for moderate use, on engines without special spec requirements.
  • Fully synthetic (100%): a lab-engineered molecule. Flows better when cold, holds up better under heat and breaks down more slowly — what nearly all current petrol and diesel engines call for, especially turbos.

Most cars under 12–15 years old now call for fully synthetic as standard. Using semi-synthetic "to save money" on an engine that requires synthetic shortens the oil's life — and eventually the engine's.

What many people don't know

What actually protects your engine isn't the brand on the bottle — it's whether it meets the API spec (SN, SP...) and ACEA spec (A3/B4, C2, C3...) your manufacturer calls for, and on engines with a particulate filter or petrol catalyst, the manufacturer-specific spec too (VW 504.00/507.00, MB 229.51, BMW LL-04...). A well-known brand that doesn't meet your car's exact spec protects worse than a lesser-known one that does.

Cheap oil or expensive oil? What the price actually buys

Between two oils that meet exactly the same spec, the price difference usually comes down to additives: finer detergent and dispersant packages, better resistance to oxidation under sustained high temperatures (motorway driving, summer, towing), and viscosity that holds steadier over more kilometres. Under normal city and road driving, within the recommended interval, that difference barely shows.

Where it does show is under demanding use: a turbo engine driven hard, constant motorway miles in summer, or cars with long manufacturer-set change intervals (15,000–30,000 km). There, an oil with a better additive package holds up longer without breaking down early.

What doesn't make sense is paying extra purely for the name on the bottle without checking the spec — or buying the cheapest one on the shelf if it doesn't meet your engine's spec. That's where the real risk is, especially on engines with a DPF/FAP, where the wrong oil can speed up filter clogging (we cover this in our DPF/FAP guide).

Common myths about engine oil

Before deciding what oil to use, it's worth ruling out a few ideas that circulate in workshops and online:

  • "Thicker oil protects better." False if it's not the viscosity your engine calls for: oil thicker than specified can make cold starts harder and slightly raise consumption, without adding any real protection.
  • "Mixing brands is dangerous." Not if both oils meet the same spec — but it's best avoided, since each manufacturer tunes its additive package differently, and mixing dilutes their effectiveness somewhat.
  • "Dark oil means dirty oil." Not necessarily: many modern oils darken quickly because the detergents are doing their job, carrying particles in suspension. What matters is mileage, not colour.
  • "Changing it early is always better." It won't hurt, but it won't fix a leak, excessive consumption or an underlying engine fault either — it just delays the symptom.

How to find out what oil your car needs

It's in the service book, sometimes printed on the engine's own cap, or in the manufacturer's technical sheet. Buying a used car with no service history? Start by checking the oil itself: clear, recent-looking oil is a good sign the car's been properly maintained. Can't find the data? Tell us the make, model, year and engine — we'll give you the exact viscosity and spec in a minute, no obligation.

What an oil change costs in Valencia (2026)

Indicative prices, labour and filter included, with fully synthetic oil meeting the manufacturer's spec:

  • Petrol, economy-to-mid range (4–4.5 L): ~€65–90
  • Diesel with DPF/FAP (specific spec, up to 6 L): ~€85–120
  • High-performance engines or premium specs (VW 504/507, MB 229.5x): ~€100–140

If a quote comes in well below these ranges, ask which spec the oil they're fitting actually meets — that's the question that separates a job done properly from a saving that costs you more later.

What we do at 2 Hermanos

We always do the oil change with the exact viscosity and spec your manufacturer calls for — not "whatever's in stock" — and check the filter, other fluid levels and any leaks while we're at it. Fixed quote before we touch anything.

Not sure what oil your car needs, or whether the last change used the right one? Message us on WhatsApp with the make, model, year and engine — we'll confirm it and give you a fixed price.