Mixing oil brands

Hey all, got a quick question for ya.

Am about to change the oil in my truck and g/f’s corolla and I have 4 quarts Castrol and 5+ quarts Mobil 1 0w20 (I just buy whatever’s on sale). G/f’s dad informes me I can’t mix different oils period. I think he’s crazy but gotta respect the man.
What do you all say?

 

Yea he's crazy.
That's like saying you have to run ONE brand of oil the entire life of your vehicle unless you disassemble your engine to completely flush out the previous brand.
You'll be fine.

 

They used to recommend not mixing brands of oil. The explanation for it I was told is due to the additives each uses potentially being incompatible. Couldn't say if that ever really was true (this was back in the 1970s and 1980s) and if it was then if it still is now. I personally avoid doing it but if I had to use whatever I could find to top off I wouldn't think twice about it.

 

It's not something I make a habit of, but when working on my 15+ yr. old vehicles, I'll sometimes have a mixture of who-knows-what-oil going back in. Never had an issue doing this.

 

As stated previously. The base oil is basically exactly the same. the additive package is really what makes them different. I don’t think there’s any additives used these days that’s not compatible. I wouldn’t make a habit of it but you’re ok to mix brands

 

I don’t think there’s anything wrong with mixing like oils ie. synthetic with synthetic, and same viscosity and so on. But I can’t say positivity. I never heard the answer from an expert such as an automotive petroleum chemist or other experts in the automotive industry. I have done it in the past but it was a temporary thing when a car was down a quart of oil and I added what was available. I doubt you’d do any harm but wouldn’t do it on a regular basis.

 

Mixing different oils within the same brand is certainly fine, it's exactly what they are doing when they make a synthetic blend.

However, doing some research I found the API has already thought about all this for a while. There's a coupe of specifications in the API standards defined as ASTM D6922 (for SN, SM) and FTM STD 791C (for SJ, SL) "Standard Test Method for Determination of Homogeneity and Miscibility in Automotive Engine Oils."

https://www.astm.org/Standards/D6922.htm

The statement of significances says:
"It is important that engine oils from different manufacturers be homogeneous and miscible with each other, because operators of automotive engines often do not have prior knowledge of the manufacturer of the oil that is currently used in their application, and engine failure can occur if oils are combined that do not stay homogeneous and function properly."

Miscibility is the technical term that describes when you mix several components together how well they becomes homogeneous. Oil and water have very poor miscibility, for example.

I don't know the details of the specification, like is there a limit to the ratio of different oils that can be mixed. IOW, is 50/50 the test or is it just when doing a change that the remaining 5% of the old oil won't interfere with the 95% new oil.

Also being compatible doesn't necessarily mean the combined oil is still going to perform the same as either individual might have.

 

Non sense, it's not going to hurt anything. Just think of all the issues there would be through the history of automobiles & not just limited to automobiles.

It's kind of like the recommenedation of sticking to one brand of fuel. As said it's the additives that different companies use.

 

It may have been at one time a concern since the API eventually started testing for it.

Came across this post on BITOG forms:
https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3503439/The_oft_quoted_ASTM_D6922

 

Only skimmed through that but yeah, I probably shouldn't have through the entire history of automobiles, I'm sure early on things were much diffferent. But that's long before my time & we're talking modern oils now.

 

Yeah, I'd think so but it was apparently still an open question in 2002 when those posts were made. I guess my $0.02 is that there may have been truth to not mixing brands in the past and that there's no guarantee still in the API certification, so I'd probably still avoid doing it if possible. It's probably not a problem but the test isn't testing each oil with every other oil. Just against 6 baseline reference oils.

 

Possibly & I agree but the point is it's not going to harm anything. Sure it may affect the optimal performance of the oils a bit but what I was getting at is it's not going to significantly cause major issues like what was possibly the concern for this post, "can't mix different oils period". There would be so many voided warranties & countless repairs required if that were the case, man imagine that!