What is the correct grade of car engine oil to use

Tuning guide "Choosing the correct grade of oil for your car."

Choosing the correct grade of oil for your car. Guide

Choosing the correct grade of oil is important. If this is too thick will cause a pressure rise. The pressure is measured by the resistance to flow between the pump and the bearings. If there is too much resistance, there will be dry spots. It takes longer to get to the valve train and does not penetrate the bearings. In tests a 20W-50 will take 4 seconds longer to reach the bearings than a 15W-40 in an engine but Toyota says you can use either. Guess which one makes better sense?

Each engine is designed to run at an optimum temperature (which varies from engine to engine) and the oil viscosity is selected by the manufcaturer to match this and the temperature is maintained by the thermostat. Using an incorrect grade of oil then is inadvisable and can cause excessive engine wear.

Ignore the terms synthetic and mineral for the time being, a 5W-20 mineral based oil and a 5W-20 fully synthetic oil have the same viscosity and will seep through the rings almost the same rate if used incorrectly. If we were to split hairs, the mineral oil would seep through more, since it will shear down more. The point could be made that "higher mileage" engines could benefit from slightly increasing viscosity from what was originally recommended (5W-20 to high mileage 5W-25 or 5W-30).

Synthetic oils when recommended should never be substituted for mineral oils. They are more stable, can last a lot longer and have lots of protective additives and ingredients. Synthetics are recommened for a reason and although more expensive the longer service interval and stability is a good trade off.

So which is the correct grade and type of oil to use in your car?

Most manufacturers recommend a specific grade of oil but sadly the SAE number is only a small part of the issue and two oils with the same number can perform dramatically differently. You should, however, be guided by the manufacturers handbook, especially if they have an approved oil type.

Performance car forums, like TorqueCars have specialist technical forums that give advice on which oils to use for your car.

It has been reported that use of incorrect oil types, especially in turbo applications (where oil is effectively baked in a hot turbo when the engine is shut off) can cause problems. Lots of short journeys and excessive idling can create a sludge build up carried by the oil and this is deposited in the engine. The sludge can act as an insulator and cause overheating and hot spots. We have also known sludge to blocks the oil ways and oil pump resulting in a drop of oil pressure and engine failure.

Engine oils have a number of additives that enhance the viscosity, prevent corrosion and prevent or reduce the risk of combustion of engine oil. These additives are critical to the quality of the oil and is the primary reason why you should always get a good quality engine oil.

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There are (primarily) 3 types of oil mineral oil, which is derived from refined crude oil, fully synthetic which is a blend of chemicals and polymers designed and engineered to lubricate the engine and then there is a semi synthetic which is a blend of the two. In reality, the mineral oil uses the polymers to cause a thin mineral oil to act like a thicker oil when hot. A 100% synthetic does not use polymers. A 5W-30 mineral oil is a 5W oil with polymers to make it act like a 30 at operating temps. A 5W-30 Synthetic is a 30W built to act as a 5W when it gets cold. 

You will get what you pay for and it is worth getting a good quality oil of the correct grade. Ask a good oil supplier for assistance in choosing and oil and stick with that brand for top ups.

Don’t get too hung up on oil grades as most multigrade oils are suitable for modern cars. As oils heat up they become more runny, the aim is to get an oil that can cope with a cold start in a frosty day and then go on to fully lubricate a hot engine. The aim is to maintain the flow rate of oil as it gets hotter it gets thinner until it fails to fully protect the metal surfaces.

A base cold start oil (the first SAE number) of 5w or 10w is more than adequate for most people in most areas, in fact you only need to consider lower figures than this in extremely cold environments.

The second figure is todo with the working temperatures of the oil. A 40 is generally adequete for most engines operating at normal temperatures. On track days, heavy driving and in high performance engines, the heat build up can be extremely high. In this case using an oil with an SAE rating of 50 makes more sense. Oil viscosity is determined by lateral force and speed. In a turbo or crankshaft bearing there is no lateral force, just velocity. The higher the velocity goes, the lower the corresponding viscosity should be. The same applies to the bearings. So if you go higher in basic viscosity then the operating temperature compensates, you create more resistance and heat, reducing performance. In race cars with Honda Vtech engines that would normally run a 5W-30, it is not uncommon to run a 5W-50. But all they do is race. No street use.

It is worth noting here that Diesel engines run at much lower temperatures than the Petrol equivalents and as such have completely different lubrication requirements.

If you allow your engine to warm up properly, by driving at low rpm bands rather than letting it idle, before driving it hard you are giving the oil an opportunity to reach an effective operating temperature. This becomes even more important in winter months. Sadly though we see many drivers using high RPM on cold engines! Then they wonder why they are faced with an expensive engine repair bill.

Oil changes are best performed on an annual basis or more frequently for high mileage and performance car drivers. Long life oils are available but we feel safer knowing that the oil and filter have been changed each year. Oil changes depend a lot on the way the car is driven. Lots of short journeys or very hard driving and the intervals should be shorter, lots of long distance cruising and gentle driving and you will get away with a longer interval.

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2 Responses to “Choosing the correct grade of oil for your car.”

  1. dawidx says:

    i drive a 2.0irs ford laser. modded to turbo charger. do you have to lift the oil pressure for the turbo , or run oil pressure standard and what is minimum pressure when you run a garret turbo with bushes? thanks

  2. CHRISTOPHER says:

    if your car consumes oil and puts out a little blue smoke on morning starts, it may be that your valve seals are worn out. 20-50 will help prevent oil leak-down after you shut off the engine because of its thickness at low temperature is higher (20).

    I use it on a ’92 corolla with 200K mi. and it makes a big difference on my oil consumption. Note, i also use Energy Release additive to prevent engine wear on start up and increase fuel efficiency.