Understanding why different viscosities are recommended?

I heard that 5W-40 was the factory fill for my 2004 WRX. Could be wrong I know. But even then Subaru allowed for a wide range of different standard oils in the owners manual including 10W-30, 10W-40, and 5W-30 as well as any number of HD and other weights for towing/high temps.
 
Yes, they "allow" for it, but recommend 5W30, just as the 6.0PSD "recommended" 10W30. I think my 01 Forester OM allows even 50 weight oils, but considering this would take roughly 150 of the 155HP the engine makes to pump something that thick (that's tongue-in-cheek BTW), it would likely result in dismal economy for no real benefit over a good 5W30 synthetic.
 
I find it interesting that neither 5W-30 or 5W-40 is any good above 30 degrees Fahrenheit.
grin2.gif
 
Originally Posted by SubieRubyRoo
Yes, they "allow" for it, but recommend 5W30, just as the 6.0PSD "recommended" 10W30. I think my 01 Forester OM allows even 50 weight oils, but considering this would take roughly 150 of the 155HP the engine makes to pump something that thick (that's tongue-in-cheek BTW), it would likely result in dismal economy for no real benefit over a good 5W30 synthetic.

I do recall that Subaru started recommending "synthetic 5W-30" for their turbo models even after they went to 0W-20 for almost everything else.

I can't find my owners manual but at least I can download it from Subaru. Here's the oil sections for the 2004 WRX:

[Linked Image]


[Linked Image]


Of course a lot of people have tried other weights with no problem. I've used Mobil 1 0W-50. I've heard some tried "German Castrol" 0W-30 in these engines. Maybe even a 15W-40 diesel rated oil back when they had gasoline ratings. I totally forgot about 10W-50. Where does one even find that?
 
I hate to say this - but recent experience has proven to me that manufacturer recommendations are made based on some fairly arbitrary interpretations of limited testing data - along with a desire to be backwards compatible across multiple engine oil platforms.

For example - certain OEM's have a tendency to still gravitate towards higher viscosity to provide cushion for durability issues that may or may not exist in current generation engines. Others are focusing more on the fuel economy benefits.

Ranges are provided because simply put, the oil put in your car when it is new is not the same 10 minutes later, 100 miles later or 10000 miles later. Between temporary and permanent shear, oxidation, use of sacrificial boundary additives, soot loading, dispersency of deposits etc oil is constantly changing. A 0WXX oil has a completely different base oil mixture (typically group III or PAO or a combination) than a 15W oil (almost always Group I or II). Additive packages are typically the same or very similar but potentially in higher concentrations or with boosters. Also every engine changes with time so the bearing clearances when new vs 10 years later could be different for every single engine. OEM recommendations try to take that into account and are almost always arbitrarily conservative.
 
That is a IH engine used in many fleet applications, school busses, tow trucks, delivery vehicles. I would venture out and say that just about 90% if not more fleets use bulk diesel 15w40 year round except for maybe Canada or certain extreme cold climates. There are tons of good choices in 15w40 and pretty much available everywhere, why not stick with the 15w40 year round?
 
Oil is oil, there is no oil that works perfect for every temperature range.
Compromises exist in a 5/40 oil when in super high temperatures.

15/40 oil will perform better simply because it requires less additive to make it more robust at high temperatures.
Its really that simple.

However, if you drive in normally cold climates, a 5/40 will perform better simply because it is more fluid, however, because of that, more additives are added to the oil to make it "thicken up" at higher temperatures and this is a compromise to make it perform like a 40 but its not a 40.

Bottom line, the less "spread" between the numbers, the better, everything is a compromise.

Will it matter? most likely not but, again, then why are we in this forum, because it can matter.

If your in a northern climate, sure, use a 5/40.

Southern climate? Use anything, you have the option of a more dedicated (less spread) specific oil for your above freezing operating temperatures.
Lower viscosity oils are simply because of the USA EPA where gas mileage (immeasurable to the vehicle owner) is more important then a POSSIBLE slight loss in engine life.
 
Last edited:
Again there's absolutely no point in using a 40w in the 6.0, the HUIE injection system will shear the oil down to a 30w in very short order. You can either use a 30w starting off or itl make its own 30w within a week or so.
 
Originally Posted by BigShug681
Again there's absolutely no point in using a 40w in the 6.0, the HUIE injection system will shear the oil down to a 30w in very short order. You can either use a 30w starting off or itl make its own 30w within a week or so.


Your statement makes it seem as if you think 30 weights are more shear-stable than 40's. If what is said about this engine is true, I'd prefer a 40 to shear to a heavy 30 than a 30 to a lighter 30 or 20. Right?
 
Originally Posted by BigShug681
Again there's absolutely no point in using a 40w in the 6.0, the HUIE injection system will shear the oil down to a 30w in very short order. You can either use a 30w starting off or itl make its own 30w within a week or so.

It all depends on the oil. Some VI improvers are more shear stable than others - something about the size of the polymers and the amount used to get to a certain operating temp viscosity.

A modern 15W-40 is not likely to shear down.
 
Originally Posted by gathermewool
Originally Posted by BigShug681
Again there's absolutely no point in using a 40w in the 6.0, the HUIE injection system will shear the oil down to a 30w in very short order. You can either use a 30w starting off or itl make its own 30w within a week or so.


Your statement makes it seem as if you think 30 weights are more shear-stable than 40's. If what is said about this engine is true, I'd prefer a 40 to shear to a heavy 30 than a 30 to a lighter 30 or 20. Right?

For whatever reason the 6.0 quickly shears anything above a 30w down and I mean very fast, once it gets to a 30w it stays pretty much at that Viscosity with very little change if any
 
Originally Posted by y_p_w
Originally Posted by BigShug681
Again there's absolutely no point in using a 40w in the 6.0, the HUIE injection system will shear the oil down to a 30w in very short order. You can either use a 30w starting off or itl make its own 30w within a week or so.

It all depends on the oil. Some VI improvers are more shear stable than others - something about the size of the polymers and the amount used to get to a certain operating temp viscosity.

A modern 15W-40 is not likely to shear down.

I've used rotella T6, amsoil SS 5w40 and Amsoil's diesels and marine 15w40 and all get sheared into a 30w within a few weeks probably sooner if you do a UOA on the oil with only a few days of run time on it. This is a very common thing with the 6.0 40w will not stay a 40w regardless of brand or composition
 
I wonder if there's any correlation between super thin oils such as 0W20 and excessive oil consumption?
 
Originally Posted by BigShug681
Originally Posted by gathermewool
Originally Posted by BigShug681
Again there's absolutely no point in using a 40w in the 6.0, the HUIE injection system will shear the oil down to a 30w in very short order. You can either use a 30w starting off or itl make its own 30w within a week or so.


Your statement makes it seem as if you think 30 weights are more shear-stable than 40's. If what is said about this engine is true, I'd prefer a 40 to shear to a heavy 30 than a 30 to a lighter 30 or 20. Right?

For whatever reason the 6.0 quickly shears anything above a 30w down and I mean very fast, once it gets to a 30w it stays pretty much at that Viscosity with very little change if any


And what do 30's shear down to?
 
Originally Posted by gathermewool
Originally Posted by BigShug681
Originally Posted by gathermewool
Originally Posted by BigShug681
Again there's absolutely no point in using a 40w in the 6.0, the HUIE injection system will shear the oil down to a 30w in very short order. You can either use a 30w starting off or itl make its own 30w within a week or so.


Your statement makes it seem as if you think 30 weights are more shear-stable than 40's. If what is said about this engine is true, I'd prefer a 40 to shear to a heavy 30 than a 30 to a lighter 30 or 20. Right?

For whatever reason the 6.0 quickly shears anything above a 30w down and I mean very fast, once it gets to a 30w it stays pretty much at that Viscosity with very little change if any


And what do 30's shear down to?

They don't for the most part, the oil in these trucks sees pressures of around 3000psi and it's rammed through a hole the size of the tip of a pen, anything heavier than a 30w gets reformulated into a 30w
 
Originally Posted by BigShug681
Who?

Well if you're saying the 40 grade is shearing down to a 30 grade, but the 30 doesn't shear then that means the base stock has the same viscosity. The oil molecules themselves do not shear.
 
Originally Posted by kschachn
Originally Posted by BigShug681
Who?

Well if you're saying the 40 grade is shearing down to a 30 grade, but the 30 doesn't shear then that means the base stock has the same viscosity. The oil molecules themselves do not shear.

For the most part the 30 grade doesn't shear down I've went 10k miles at a heavy 30 compared to dropping after 5k miles from a healthy 40 grade. I always run out of tbn before the oil gets down to a mid 30 grade
 
You must log in or register to reply here.

Similar threads

Camperguy
3.6 Pentastar viscosity done by clearance and temperature
2
Replies
32
Views
2K
kschachn
F
Why do people say weight when it’s viscosity?
2 3 4
Replies
76
Views
4K
MSCH
H
Oil Recommendations for Toyota Fortuner (South Africa)
2
Replies
24
Views
4K
heever91
3UR-FE
2022 Camry 2.5 fuel dilution/viscosity selection question
2
Replies
27
Views
739
ATex7239
P
Gx460 viscosity
Replies
11
Views
2K
Lavrishevo
Back
Top