Is it OK to mix oil of different viscosity? Is it likewise OK to mix synthetics with petroleum based oils?

For example: Mix 2 qts, of 5W-30 with 3 qts. of 10W-40. What will be the end result in the viscosity of the mixture. Why would I want to do this> Because I have too many of each. As regards to synthetics, the oil manufacturers stated that it’s OK to mix synthetics with pertroleum based oils. After all the semi-synthetics such as the Syntec Blend is such type oil.

11 Responses to “Is it OK to mix oil of different viscosity? Is it likewise OK to mix synthetics with petroleum based oils?”

  1. pbleek Says:

    All oils are compatible, thanks to the US military who established standards which make it so. You simply end up with a viscosity somewhere between the two oils and if you add synthetic you have a blend.

  2. Stampy Skunk Says:

    It’s not ok to mix different viscosity’s and not ok to mix synthetic and conventional motor oil….. don’t mix these up….

  3. Kevin S Says:

    No do not mix different weights of oil especially that different of weights. See the weight refers to what it is at startup and what it is at running temp. One screw up like that may not cause long term damage but if you were to do it alot it will mess up the motor.

  4. fordman Says:

    I don’t think that is too good of an idea. I would want the oil all the same. I don’t think that it will really make a difference, but it is just good form to have the same oil in the crankcase.

    As for mixing the synthetics with regular oil, then you are losing the benefits of the synthetic with the higher temps. I wouldn’t do it. But that is just my personal opinion.

    good luck.

  5. mdcbert Says:

    It will be fine! Synthetics can also be mixed in too as they make synthetic blends for cars.

    Oil Viscosity
    The viscosity of multi-weight motor oil is specified using two numbers. The first number is the viscosity when the oil is cold. This is followed by the letter W (which stands for winter, not weight), which is followed by the number that indicates the viscosity when the oil is at operating temperature. The higher the number the thicker the oil.

    5W30 versus 10W30
    Virtually all new passenger vehicles sold in the U.S. use either 5W30 or 10W30 oil. The difference between the two is that the 5W30 flows better when cold, so if you live in a cold climate or operate your vehicle in a cold climate during the winter months, you should use 5W30 if it is the preferred oil for your vehicle. If you live in a sub-tropical climate and don’t operate your vehicle in cold climates, then 10W30 is acceptable as long as the manufacturer specifies that it is permissible to use it.

    Is there a disadvantage to using an oil that flows better when cold, i.e. 5W30 versus 10W30?
    Sometimes, but usually not. The crux of the issue is this: the bigger the difference between the cold oil viscosity and the hot oil viscosity, the more the volume of viscosity modifiers and the less the volume of base stock. If you are good about following the manufacturer’s recommended oil change interval then stick with the 5W30 if that is the preferred oil for your vehicle, even if 10W30 is acceptable in warmer climates. Older cars may specify 10W30 only. This is because they need a little more viscosity when cold to keep a protective film on the cylinder walls. There have been instances where the larger amount of viscosity modifiers that are present in 5W30 have broken down due to excessive heat and have left carbon deposits on the valves, but this is extremely rare. The proper fix would be to reduce the excessive heat, but the workaround was to use an oil with less viscosity modifiers.

  6. wuddy12 Says:

    NO. . . . Do not mix either. It is bad for your seals and gaskets. It will cause leakage the would require a rebuild.

  7. wrenchbender19 Says:

    well since all oil is compatible you can mix the different viscosity if you would like and the whole idea of conventional and synthetic is fine to mix especailly if you are converting all the way to synthethic from conventional which you have to blend together for a few oil changes before you go full syntethic there is nothing wrong with this if you are only doing this one time either

  8. boom_boom_taz69 Says:

    Yes, you can mix oils of different weights as well as mixing petroleum and Synthetic. As for the result, it would be roughly a 8W-35. Mixing petrol and synth will give you some of the benefits of synthetics with a lower price tag. The main thing is to make sure you use a quality filter and regular oil change intervals.

  9. whateveryouwant Says:

    yes you can. but why? It makes more sense to run one or the other. unless you trying to get a desired viscosity.

  10. sanjeev Says:

    You have provided really useful information.I loved the way you presented your ideas.It is really informative.

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