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

6457 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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Automakers want to maximize fuel economy, and will recommend the lightest oil that will do the job. My company car is an '07 Explorer.... uses 0w20 year-round.

Personally, I run 5w30 in the wife's cage.
Theoretically, 10w30 will hold it's vicsosity better than 5w30, but at the expense of cold-flow, and running 5-10k change intervals, viscosity breakdown is not going to be a problem with a quality synthetic in a cage.
0w20 is recommended even in hotter climates? That's pretty bold of Ford.
That's Ford... the company that brought you the recommended inflation level of 28psi on an SUV wearing tires designed for 35psi.
Oh well... It's a company car leased from Enterprise. If it breaks, Enterprise gets to fix it for the next 2 years, then the company will likely buy out the leases and it becomes their problem. If I show up at a job site on the Vulcan, my boss will know why :biggrin:
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.
Synthetic is more stable period because it does not require the large levels of viscosity improvers that conventional oil requires.
20w50 is more stable because the ratio between 20 to 50 is 2.5:1. 10w40 is 4:1.
It's not the "number of points" that makes the difference, it's the ratio between low and high.
Now back to synthetics.... Due to their need for less viscosity improvers, a quality 5w40 synthetic (8:1) can perform as well as a 15w40 conventional (2.7:1) Those are the two oils specified for most consumer Diesel engines.
Anywhere that the temperature can break 100 degrees and I'd be running 20w50 in the summer anyway.
For a bike, absolutely, especially with a wet clutch and combined engine/tranny oiling system, and double-especially for an air-cooled bike.
Liquid cooling helps stabilize operating temperatures, but 20w50 is still a good idea if bottom end temps don't dip below 40 very often.
Keep in mind that on the V2K, only the heads are liquid cooled.

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.

I do run 10w40 in my dirtbike, primarily run it in ambent temps between 50 and 90.
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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.
That is the shear stability that is the primary concern of those running oil analysis reports.
I've had 20w50 products go 2,000 miles in my Vulcan and remain in grade for a 50wt, and I've had products go 2,000 miles and come out as a heavy 30wt.

Amsoil and Mobil-1 are good products, though the Mobil-1 automotive oil that I tried (as well as Royal Purple automotive) both sheared down almost as badly as Castrol GTX. By comparison, both Amsoil MCV and Mobil-1 V-Twin remained in grade at 50wt.


As I mentioned before, shear stability is a primary concern in our bikes with their wet clutch and common engine/transmission oiling system. Those systems are very hard on oil, especially the gearbox.
In a cage, the hardest thing on the oil are the cam gears in a DOHC engine. Shear stability is not THAT big of an issue in cages... additive package durability and contaminant suspension are the primary concerns there.
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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.

Jiffy Lube pumps more 5w30 dino than anything else. Every cage that I've bought in the last 10 years has spec'd 5w30, not specifically calling for synthetic.
Correct... they do offer options based on temps, but due to fuel economy standards, they push for the lightest that'll get by.
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.

Note the zinc levels between the first 3 (MC specific) and the last 3 (automotive)... I didn't think to check the API rating on the GTX 20w50 or the Royal Purple, but the Mobil-1 15w50 was SM.
That and the moly also came out high. Blackstone was very surprised by it, and even retested both virgin and used samples AND recheck the calibration on their equipment. Their only answer was that is must have been somewhere in the engine from the previous oil.
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