Can I mix oil viscosity?
10-04-2013, 06:18 PM
#11
Somewhat wrong - and wrong.
It is not really a good idea to mix viscosities. But what does that mean? Well, it depends on the oils and how well the manufacturer actually adheres to viscosity ranges. The better question is: can I randomly mix 2 different oils? The answer is: It's not a good idea - certainly not from a regular practice standpoint. Emergency/temporary situations are a different thing altogether.
Here is an interesting example from a different perspective: I currently use Amsoil ZRT 10W30. I would not actively mix Amsoil ATM (also a 10W30) because the chemistry is different. It's not that they are not "compatible", it just not optimal and deliberately doing so is not a good idea.
Now if I was running Amsoil ATM (10W30) and I needed to top off, and I only had a quart of Amsoil ASL (5W30) or Amsoil AZO (0W30) - I would have absolutely no problem doing so because all of the Signature Series 30 weight oils have VERY similar chemistries and qualities. They are different weight oils - but not significantly so. Okay - how about mixing with Amsoil 10W40? I would say no, because the viscosities and chemistries are very different.
Mixing different brands is not a good idea either. Manufacturers almost always state their oils are "compatible" with other synthetic or conventional oils. However, there is not a single oil manufacturer that has tested their oil chemistry with other brands of oils. Foaming is one of the bad things that can happen when oil chemistry does not play well. Foaming is bad.
You "can" mix synthetic and non-synthetic - but again, there are differences between the chemistries and therefore it is not a great idea.
There can actually be potentially bigger problems with actively mixing different types of synthetics (Group 4 and Group 5 synthetics), and I would never do so unless I was fully changing from one to the other.
Which brings me to my final point: when you change from one brand oil to another, from one type of oil to another, and from one weight to another there will be somewhere around 10% of the old stuff left in the engine. That is why I am a big proponent of only going 1000-2000 miles on a change when you have gone from one type (weight, brand) of oil to another. This prevents problems. The only exception to this rule of thumb is the Amsoil Signature Series 30 weight oil example I gave before - and you feel you need to change from a 10W30 in the summer to a 0W30 in the winter.
Also, don't constantly change types//brands of oils (the penny pincher that just goes for the cheapest bargain at every oil change).
If you need to mix oils because of a genuine emergency - then run something that is as close to the oil that should be in the engine as practical (to get you where you need to go), and then change the oil as soon as possible. Mixing motor oils in an emergency situation for a short duration is not going to cause any major problems.
It is not really a good idea to mix viscosities. But what does that mean? Well, it depends on the oils and how well the manufacturer actually adheres to viscosity ranges. The better question is: can I randomly mix 2 different oils? The answer is: It's not a good idea - certainly not from a regular practice standpoint. Emergency/temporary situations are a different thing altogether.
Here is an interesting example from a different perspective: I currently use Amsoil ZRT 10W30. I would not actively mix Amsoil ATM (also a 10W30) because the chemistry is different. It's not that they are not "compatible", it just not optimal and deliberately doing so is not a good idea.
Now if I was running Amsoil ATM (10W30) and I needed to top off, and I only had a quart of Amsoil ASL (5W30) or Amsoil AZO (0W30) - I would have absolutely no problem doing so because all of the Signature Series 30 weight oils have VERY similar chemistries and qualities. They are different weight oils - but not significantly so. Okay - how about mixing with Amsoil 10W40? I would say no, because the viscosities and chemistries are very different.
Mixing different brands is not a good idea either. Manufacturers almost always state their oils are "compatible" with other synthetic or conventional oils. However, there is not a single oil manufacturer that has tested their oil chemistry with other brands of oils. Foaming is one of the bad things that can happen when oil chemistry does not play well. Foaming is bad.
You "can" mix synthetic and non-synthetic - but again, there are differences between the chemistries and therefore it is not a great idea.
There can actually be potentially bigger problems with actively mixing different types of synthetics (Group 4 and Group 5 synthetics), and I would never do so unless I was fully changing from one to the other.
Which brings me to my final point: when you change from one brand oil to another, from one type of oil to another, and from one weight to another there will be somewhere around 10% of the old stuff left in the engine. That is why I am a big proponent of only going 1000-2000 miles on a change when you have gone from one type (weight, brand) of oil to another. This prevents problems. The only exception to this rule of thumb is the Amsoil Signature Series 30 weight oil example I gave before - and you feel you need to change from a 10W30 in the summer to a 0W30 in the winter.
Also, don't constantly change types//brands of oils (the penny pincher that just goes for the cheapest bargain at every oil change).
If you need to mix oils because of a genuine emergency - then run something that is as close to the oil that should be in the engine as practical (to get you where you need to go), and then change the oil as soon as possible. Mixing motor oils in an emergency situation for a short duration is not going to cause any major problems.
10-06-2013, 02:31 PM
#12
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On my oil changes I use 1 quart of 10w30 Castrol high mileage and the 4+ remaining Castrol edge 10w30. Been doing this for a few years now after reading about it on bobistheoilguy forums. A lot of lubrication engineers were doing this to gain the added conditioners in the high mileage oil while still using an 80% full syn and the 20% high milage blend.
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