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

6459 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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0w20 is recommended even in hotter climates? That's pretty bold of Ford.

I've read that 10w40 is more stable in synthetic formulation, and 20w50 is also more stable than 10w40 as it's thicker oil. Apparently thicker oil is less affected by viscous breakdown.

Anywhere that the temperature can break 100 degrees and I'd be running 20w50 in the summer anyway.
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 think it might be because, as oil technology improved, auto manufacturers realized how quickly 10w40 mineral breaks down, and started "outlawing" it for use in their engines. Don't most new cars that call for 5w30 also call for synth?

Although that doesn't really make sense as 10w40 is still the recommendation of the vast majority of bike manufacturers. And I'd think a motorcycle engine is going to work any oil harder than a comparative car engine. That's a very good question bountyhunter!
Newer oils have different additive packages that older oils don't. I know for one, newer oils have all but dropped zinc from the additives because it interferes with the catalytic converter (to the point of damaging it, as explained to me). So an older rating like SG has xx amount of zinc in it, and SM has less as its for newer cars.

I think SG is the dino Castrol m/c I use, it can't be rated anything newer even though SG is a 25 year old standard. It definitely has a zinc additive (ZDDP?). As such, it could damage a modern car and because you used an "incompatible" oil the manufacturer wouldn't be required to warranty the damage.

Most bikes are fine running older oils, although they're kind of rare to find. As far as I know this zinc example is one of the rare reasons to justify the rating system on oils, other than the fact that I'm sure a 10w-30 from 1956 isn't going to perform as well as one from 2006.
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Haha it seems your 15w-50 grew some zinc... It's supposed to form a thin layer around metal, I wonder if the zinc you gained in the last 3 oils after use was from the original Mobil 1 v-twin formula?
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