Can/could one mix motor oil viscosities/brands w/o engine damage?

Factual Questions
1

In the 60s, and 70s, when I was coming of age auto-repair-wise, it was common knowledge that mixing oil viscosities would damage your car’s engine. Never knew why, though, but everybody said that. I never believed it, but at the same time, I never went out of my way to disprove it. Then, in 1983, I had a boss whose wife was taking an automotive repair class at a local community college. She told me that her instructor told her that mixing viscosities was fine, one just couldn’t mix oil brands! Just great. No wonder my Corvair wasn’t doing just fine, thank you! (No Corvair, just joking.)

So, were they all wrong, or was/is there something to that concept?
If so, has the mechanism that made that true changed in the ensuing years?

Thanks,
hh

2

Mixing Viscosity shouldn’t be a problem. - That said you’re engine is designed for a certain viscosity.

Most Oils contain additives for cleaning your engine among other things. There is no surity that mixing different companies additives will be safe. Odds are you’ll be fine. I’ve never worried about brands when adding oil. I have my preferences, but If they don’t have my top choice in brands, I’ll take the next best thing. All brands of oils meet the same minimum standards.

According to Mobil’s website you can even safely mix synthetics and petroleum oils - They say it should be fine, though they recommend against it.

3

Are you sure? I’m really curious as to how you would design an engine for a certain viscosity. I’m not (intentionally) being being an ass. However, I would think it would be just the opposite… Once the engine is designed, and overall heat, and other measurements would lead to a recommended viscosity. What’s different from one engine to the next that says how much viscosity is required for a piston ring to rub against the cylinder wall that could be engineered?

4

I actually work for an engine oil additives company… but am on the production side, not the setting-specs side of the business.

That said: Most engine oils you’re going to buy off the shelf will have the same basic group of additives – each company has its own formulation to be sure, and some companies prefer one type of additive over another for various purposes, but they’re all basically detergent, dispersant, antioxidant, viscosity improver, and base oil.

They’re “safe” to mix, but each company would state that you won’t be getting their optimum performance. If you’re going to mix them, I’d recommend not mixing grades (although my lab tech “uptreats” an inferior grade with a superior grade, I’d say just buy a better grade). I, personally, prefer synthetic but don’t think there’s much problem in mixing the two. I’m only brand loyal because I know which brands my product is in, otherwise I’d buy by grade.

Specs for engine oils are set by the industry, but basically the car manufacturers are the ones who set the requirements, based on the performance they want to have for their cars. They are the ones setting the viscosity requirement, and yes it is based on the engine performance, not vice-versa.

The various “oil companies” – Valvoline, Quaker State, etc. – then purchase additives or additive packages that will meet these specifications once blended with the oil company’s base oil.

Additives companies, like where I work, actually manufacture the additives and run the engine tests to verify that the additive package will pass the car company’s specification requirements.

I’m not famliar with the older history of the industry, so don’t know whether mixing oils used to be an issue. I suspect if you had an engine that needed a high-performance oil, and you mixed in some untreated oil, the lack of additives could’ve wrecked a high-performance engine. Nowadays, even the lower grades of oil have to meet higher standards, so unless you’re mixing motor oils from the Third World you’ve nothing to worry about.

5

If you look at your owner’s manual, there is probably a little chart of which viscosities you can use at what outside temperatures. You’ll notice that for many climates, there’s several viscosities you can use. Looking at this chart, you can infer that the car requires at least x weight hot viscosity for x high temperature and no more than x weight for x low temperature. So long as the mixed viscosities still meet these critera, it should be hunky dory.

These days, most car companies just choose a viscosity that’s good for most climates (usually 5w-30), but there’s nothing magical about that exact viscosity.

That said, with the way multi-weight oils work, I don’t think there’s a reliable way to predict what viscosity you’ll end up with by mixing them. So I wouldn’t say for example “My car takes 5w-30, so I’ll just put in two quarts of 0w-20 and two quarts of 10w-40” and expect the result to be exactly 5w-30 motor oil. But just topping off with another reasonably close viscosity should be fine.

6

Way back when (1940’s?), detergent oil was introduced. It was a big improvement over non-detergent oil, except…adding the new detergent oil to an engine that had been run for a long time on non-detergent oil sometimes worked too well and loosened deposits that hadn’t caused trouble where they were, but now got into circulation and caused engine damage. The warning to not thus “mix” detergent oil with non-detergent oil mushroomed into cautions about mixing any different types (brand, viscosity) of oils.