What do the numbers and letters in a motor oil specification mean? For example:




Which values are better? Also, sometimes there are additional letters and numbers afterward like:

5w-30 C3

0w-40 A3/B4

When choosing which to use in a car, do the small differences mean much? These last two are specifically the ones Castrol recommended for a VW GTI 1.8T Mk4 (180hp), but your answers don't need to be specific to that car.


3 Answers 3


As far as the numbers, use what your vehicle manufacturer recommends. There is a reason your manufacturer recommends a viscosity, the main reason is, the engine is built a certain way and needs that viscosity. If you put a heavier weight oil in the engine than what is called for, your engine will not get the oil in a quantity it needs, nor in places it needs it. It also drags the engine down some and causes additional wear/stress on the internal engine parts which run the oil pump. If there gets to be too much pressure at the relief valve, some of the oil is bypassed and is not being used to lubricate the engine. None of these things are good. If you put a lower viscosity oil in then the engine was designed for, your engine will lack the pressure it needs to get the oil to all the places it needs to be. For both of these scenarios, there is a domino effect which will increase wear. You may not see it right away, but it will affect the longevity of the engine.

As for what do the numbers mean ... this post should explain it all.

  • Hmm, I had tried searching first and didn't find that question asked already, so mine is probably a duplicate.
    – MasterHD
    Commented Apr 18, 2015 at 20:02
  • @MasterHD - Mostly a duplicate ... I obviously thought the lead-in paragraph was needed, though. Commented Apr 18, 2015 at 21:54
  • 1
    as for what the numbers mean, THAT POST did explain it all
    – chilljeet
    Commented Apr 23, 2015 at 6:25

As the previous answer stated, always use the oil specified in the owners manual for your vehicle based on temperature.

As for the numbers and what they mean, the numbers are the "viscosity" of the oil. Viscosity refers to the thickness of the oil, lower numbers are thinner, higher numbers are thicker. More appropriately the viscosity number is the amount of a given oil that can flow through a given oriface within a fixed amount of time which determines the viscosity number. The lower the number the more easily it will flow. This test is done when the oil is both cold and hot.

The numbers of a typical multi-weight oil have specific meanings. As stated above, viscosity numbers are taken for oil when cold and hot. This is what the numbers mean. The first number is the viscosity of the oil, how it will flow, when cold. The second number is the viscosity, how it will flow, when hot. So that means a 5w-20 motor oil will have the viscosity characteristics of a 5 weight motor oil when cold. It also means that it will have the viscosity characteristics of a 20 weight when hot. You might be asking what does the "w" mean. That means it is appropriate to be used in the winter.

The reason oil pressure is important is that the clearances between bearing and crankshaft allow for pressurized oil to flow into those thousands of an inch and be the only thing to keep them from making contact. If it were not pressurized the constant push from the power stroke would push the oil out until contact would be made and catastrophic failure would be the result.

For many years engine oil was simply "30 weight", or "40 weight". The only option was a single weight oil. It would thin as it would heat up, so you had a thick oil when cold and thinner when hot. Just the opposite of what an engine needs. Because of technology, scientists learned how to mix the oil with additive packages to allow an oil to be have different viscosity characteristics that change with temperature. This allows the oil to actually maintain its flow throughout varying temperature ranges to assure the constant flow of the life's blood of an engine, it's oil.

As for the newer addition at the end of the designation, these are new technologies that are additives with various characteristics. One is magnetic to allow it to "stick" to the internal parts and not leech out during the night, so in the morning when you start the engine it still has lubrication until the oil pressure builds. Most engine bearing wear is occurs when the engine is first started and there is no oil being pumped to the bearings. That is the time the bearings and the crankshaft could come in contact and wear occurs.

I hope this answers your question.


The first number represents the viscosity at low temperature. The second number represents the viscosity at 100C(212F). The W represents winter.

The API service classes have two general classifications: S for "service/spark ignition" (typical passenger cars and light trucks using gasoline engines), and C for "commercial/compression ignition" (typical diesel equipment)-Wikipedia

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