Mixing oil

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1,307
Location
PEARL River la
Getting close to oil change on my Caravan so I will be using 5w20 Valvoline Advance Synthetic with Maxlife 5qt jug and 1qt of Valvoline Advance Synthetic 5w30. I have 5w30 PP in now and running good. Do you think it would have any effect on the 20 grade as both have same additives? I know Maxlife advertised more seal conditioner but that's all I see. I got these on sale with the 5w30 goes in lawn equipment so would like to use it instead of sitting on shelf. I have more Maxlife but doing this would allow me to keep Maxlife sealed. I don't even think it would cause oil to be heavier.
 
Messages
13,102
Location
ROCHESTER, NY
Why do people have a concern about mixing oils?

Because we hear too many stories on the street from those who really don't know. Even from reputible sources who say... "you shouldn't mix oils and you can't go back to conventional oil once you've used synthetic". And, "once you've done this, you can't do that".

I have a story about an AutoZone experience when I was talking to one of their counter persons who told me all of those things above. Stating that this is what they learn in their training.

I proceeded to tell this person the truth and show them one of their own AutoZone YouTube Videos which said the contrary. The counter person called over the AZ store manager to watch the video(on my tablet computer) and was dumbfounded to say the least.

This is the video I showed them at AZ. At 4:03 is where the AZ rep states that you can change between conv, hm, sb & syn at anytime.

 
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Messages
44,665
Location
Ontario, Canada
According to API oils must be able to mix with a no harm policy. But there is no guarantee of what happens to the viscosity and additive balance.

Correct, and the standard is extremely basic, this is not a complex synergies test. As Shannow used to note, it's basically a test that guarantees that when mixed with any of the standardized "test" oils the resultant product doesn't split like mayonnaise.

The biggest risk with mixing is the impact on the Winter rating. Different PPD dosage rates necessary for different base oil blends (and to some extent the same for VII treat) means that it's possible that a 5w-30 mixed with a 5w-20 puts you at a 10w-30 or even a 15w-30 or 10w-20 or 15w-20...etc depending on the ratio of the oils mixed and interaction of those components. A majority PAO lube with next to no PPD's mixed with a Group II+ one that's chalk-full, the resultant base oil blend may require more PPD's than is present in the mix is a hypothetical example of one such scenario (not sure how that aligns with reality, one of the resident tribologists could better advise on the specifics of such a scenario).
 
Messages
1,452
Location
California
According to API oils must be able to mix with a no harm policy. But there is no guarantee of what happens to the viscosity and additive balance.
This is a good policy. Many owners switch brands during a service, either knowingly (through quicky lubes) or knowingly (through doing their own maintenance). Obviously any residual oil in the engine will be mixed with the new fill, although it's often thought of being 100% the new fill, which it is not.
 
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