What is the point of 5w-40

[salgra]

Hi All,

As the title.
Most 5w40 oils have a HTHS of 3.5 3.6 some 3.7 .
5w30 A3/B4 and or C3 oil are also 3.5.

What is the advantage of using an oil that has a higher kinematic viscosity, but the same HTHS.

Seems pointless to me.

I can understand that a HDEO 5w40 having a hths of 3.9 or 4.0 can have a bennefit. And the relation of kinematic viscosity to HTHS is more or less the same as a 5w30.


Please help me understand.

thx.


this one for example
http://www.mpmoil.nl/pdf/tds.php?code=05000&action=show_and_create

I called them, it's HTHS is 3.6. But has high kinematic viscosity.

 

[VeryNoisyPoet]

Higher kinematic viscosity will create more pressure for the same volume pushed by the oil pump. This becomes important when oil temperatures are high, which otherwise could cause unacceptably low pressure to replenish bearings fast enough under high load. HTHS is important for cylinder walls where there is a super thin layer of oil under extreme heat and pressure, but bearings require a thicker cushion of oil with enough volume to avoid metal to metal contact.

Low pressure can also affect VVT and other systems that used oil as a hydraulic power source.

 

[Patman]

This was the same line of reasoning I had when choosing between two of Mobil 1's ESP Formula offerings, their 0w40 and 5w30. The 0w40 has a viscosity of 12.9 cst at 100c, vs 12.1 for the 5w30, but yet the 5w30 actually has a higher HTHS of 3.58, vs 3.53. I still don't quite understand how that's possible unless it simply points to a better base oil in the 5w30?

 

[SonofJoe]

It's actually quite tricky to make any 30 weight multi grade oil with 3.5 min HTHS. With the max allowable KV100 of a 30 grade being 12.5 cst, you sort of run out of KV100 'head room'. Put another way, 3.5 HTHS & 12.5 KV100 tend to want to occupy the same viscosity space.

The fundamental problem is compounded by base oil selection (making the grade with Group II makes life infinitely worse,) by VII shear stability (high SSI is worse) and VII type (OCP is usually worse).

However it's very easy to make 3.5 min HTHS if you can nudge the KV100 up to say 12.65 cst. Of course, this takes the oil from being a 30 weight to a 40 weight oil, which sort of explains the genesis of Euro 5W40s.

Hope that helps.

PS, you're quite correct in observing that there's often not that much of a difference between a 3.5 HTHS 5W30 and a 5W40. It's because there isn't. In fact I suspect that if someone actually bothered to check, a lot of commercial 3.5 HTHS 5W30s are at best 3.45 cP (rounded up 3.5) and often not even this. HTHS isn't a routinely measured QC and you sort of only need one 'passing' result in the candidate data package to make your claim.