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5W-30 GOOD, 10W-40 BAD? Both are Rated SM?

6460 Views 16 Replies 4 Participants Last post by  RichLockyer
OK, I have been trying to follow the various oil controversies: the latest is that 10-40 is BAD and new car makers are warning not to use it (we should use 10W-30).... the reason?

10W-40 has more polymer viscosity enhancers to make the oil shift 30 points from 10W (cold) to 40W (hot), compared to 10W-30 that only shifts 20 points. The oil is the lubricant (not the viscosity enhancers) so you want the one with less of the viscosity enhancers...


My Chevy Cavalier says to use 5W-30, which is "25 point shift". Is there really a significant difference in the volume of oil in comparing 10W-30 and 10W-40 or 5W-30?

My owners manual recommeds the 5W-30 because it's operating range allows outside temps down to about -20F. 10W-30 or 10W-40 goes down to about 0 degrees F. Not that I care living in california.

Anyway: what % of the volume of a quart of "oil" is made up of the viscosity polymers?


Why is it every quart of oil I have in my garage which are: full synth (10W-30), synth blend (10W-30), Castrol/valvoline conventional oil 10W-40, all have the same API service rating of "SM" which I believe is their highest rating?

Are new car makers actually saying that oils rated at the highest service rating are not good enough?
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For a cage, I stick with the grade recommended by the manufacturer... which has been 5w30 for the Toyotas that I've owned for the last 10 years.
And yet, that is a ratio of 6:1 which is a lot worse than 10W-40 which is only 4:1. That's why I am confused.
I've read that 10w40 is more stable in synthetic formulation, and 20w50 is also more stable than 10w40 as it's thicker oil.
No question it's thicker oil: 10W-40 is basicalle 10W oil with viscosity enhancers added, and 20W-50 starts with 20W oil. After the oil gets beat up bad enough, the enhancers go away and it ends up being whatever oil it started out as.
Nahh... the automakers have dropped to 5w30 (and lighter!) because of EPA CAFE standards. Modern engines also have tighter tolerances, so the lighter oils do a better job than in days past.
In my owner's manual, it's because of temperature. The 5W-30 is OK down to a temp of about -20F or so, but the 10W-40 is only good down to about ZERO up.

So, if you have to spec just ONE oil for the entire country, they spec the one that is barely good enough cold in Wyoming and barely good enough hot in Arizona.

They don't trust people to be able to select the right oil between two or three choices..... 5W-30, 10W-40, and 20W-50. They figure it has to be stamped on the oil filler cap (it is) so they have to pick one value.
It still leaves unanswered my original question:

Every can of oil I own from the highest grade pure synthetic down to the Chevron Supreme stuff (because it was on sale and I put in in our old beater Saturn)...

All this oil wears the highest API service rating of "SM".

How is it car makers can then say this one or that one will void the warranty?

If they are going to pull that baloney, the ratings are worthless.
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