All my life, I have heard that you should never mix brands and/or weights of motor oil. Many times on this board I have seen suggestions to do such a thing, INCLUDING mixing synthetic with conventional motor oils. Is this really an OK think to do?

As a general rule, it should be no problem mixing base stocks of the same or similar weight but keep in mind that the oil also has an additive package. In some rare cases, the additive packages of different brands may not be fully compatible. This was more true with some of the older oils of the 70's and 80's and is probably pretty unlikely today, especially with oils that meet the GF3 standard.

In any case, I would say it's the best idea not to mix oil brands or weights just to be safe. On the other hand, if you have to add a quart or so between oil changes and your favorite brand isn't available, go ahead and add a quart of a what's available. It's not a big deal.

Otherwise, I can think of no sane reason why you would want to intentionally mix brands or weights.
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Greg P. 1988 240 DL "Olga" 165,000 miles

each oil additive package is tailored to its own needs. when you start mixing stuff around, the results can be unpredictable to say the least. your car won't blow up on you, but the oil will be less than optimal.

dangerous? no.

suboptimal? yes.
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

As far as I know mixing will work but there's lots I don't know. Engine oil is a mixture of many things, there are the lubricants, viscosity improvers, detergents, anti-corrosion chemicals, and probably more. I think the lubricants will mix but maybe the others won't always work together. Here is a web site where they really get into discussing oil:

http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=forum;f=5;DaysPrune=1000

Many major manufacturers sell "blends" of synthetic and dino/mineral/traditional oil. That suggests that it's ok to mix blends. Were it unsafe, some %!*# attorney would file a class action law suit and make themselves rich. In short, I think it's perfectly fine, but have an alternative idea:

why don't you just change your oil whenever it appears dark? You're not a race car driver, and protecting your engine is paramount. More frequent oil changes with dino oil is better than less frequent changes with synthetic. Mechanics I've talked to have NOT been impressed with synthetics for daily drivers.

I've owned numerous Volvos and non-Volvos over the years. I change my oil every 3 months or 3,000 miles. Every time I've had my engine examined (eg changing gaskets), I've been complimented as to how well maintained the parts are.

Well, think about it. Oil must be compatible.
I have never, in over 25 years of peddling AMSOIL, heard of anyone who had a problem caused by mixing oils--brands, viscosities, or base stocks.
It is true that changing brands of oil, or from non-synthetic to synthetic, can result in deposits being cleaned up. This can lead to the discovery of pre-existing bad gaskets or seals--. Because the non-synthetics are so much better now than they were 20 years ago, this has not been a major problem in the past 5-10 years.

No. You will end up with a lower viscosity oil than if useing the same brand.
The detergents and chemicals will clash when mixin types/weights and the oil will be crap.
--
Kevin * HONDA spanking,1985 240DL * VOLVO ON!!!!