High Mileage 10w40: Castrol GTX or Mobil 1?

[Robvette]

I've mostly been using Mobil 1 HM 10w40 with good results (low wear and reduced oil consumption) and I've been assuming it to be superior to Castrol's GTX HM 10w40 by virtue of Mobil 1 being a full synthetic vs Castrol being a synthetic blend. Is that a safe assumption? I did notice that V@100c of the Castrol is 15.6 vs 13.9 for the Mobil 1. Any thoughts or insights appreciated. Thanks!

Here are the data sheets: GTX High Mileage 10W-40 | Mobil 1 Product Data Sheets (PDS)

 

[Urshurak776]

What is your OCI? If 5K, either will be fine. If 7500-10K, go with the Mobil 1.

 

[twX]

One reason an oil might need a high KV100 is so that it stays in grade after the shear test required by API SP. This is probably the case with the Castrol, since a 10W-40 conventional or synthetic blend oil is going to need a lot of VII, and it won't be using the expensive stuff.

Synthetic base stocks have a much higher viscosity index than conventional base stocks, so they don't need as much VII, and will usually be more shear stable.

The Castrol is allowed to shear from 15.6 cST to 12.5 cST in the shear test, or by 25%. The Mobil wouldn't be allowed to shear by more than 10% due to its lower initial KV100.

I'd tend to prefer oils with a lower KV100 for the grade, since at least then you know that they're shear stable and that the HTHS won't end up a lot lower. The Castrol could, in theory, end up with an HTHS of <3.1 cP after the shear test, assuming it starts at only 3.5 cP and barely passes the shear test. On a long OCI, it could get even thinner, since the shear test used for API SP isn't very severe.

 

[GMBoy]

I will always choose the full syn over a semi syn. That semi-syn could be 50/50 or 20/80 or whatever ratio they want and still be a semi-syn. The price difference also is usually so small it doesn't matter.

 

[SR5]

One reason an oil might need a high KV100 is so that it stays in grade after the shear test required by API SP. This is probably the case with the Castrol, since a 10W-40 conventional or synthetic blend oil is going to need a lot of VII, and it won't be using the expensive stuff.

Synthetic base stocks have a much higher viscosity index than conventional base stocks, so they don't need as much VII, and will usually be more shear stable.

Not fighting, just checking. But I thought the stay-in-grade viscosity shear test was part of ACEA (Euro oils) and not part of API. Has it recently been added to API SP?

I do agree that a full synthetic like M1 will have higher VI base stock and so should require less VII than a synblend.

The M1 would be my pick too.

 

[wlk]

Nothing wrong with the Castrol, I've ran it a couple times for cheap oil changes when towing in my Expedition. And had an Ascender w the 4.2l and 4.0l Cherokee that used it for several oil changes, nothing fell apart and ran well when I sold them at around 180k miles.

 

[twX]

Not fighting, just checking. But I thought the stay-in-grade viscosity shear test was part of ACEA (Euro oils) and not part of API. Has it recently been added to API SP?

It's been part of the Sequence VIII test used by API for a long time. It measures the viscosity after only 10 hours in an engine though, whereas ACEA uses the 30-cycle Bosch injector test, which is probably a more severe test.

 

[OilUzer]

I've been brainwashed to think:

Advanced Full Synthetic > Synthetic Blend

And

Triple Action > No Action

 

[SR5]

It's been part of the Sequence VIII test used by API for a long time. It measures the viscosity after only 10 hours in an engine though, whereas ACEA uses the 30-cycle Bosch injector test, which is probably a more severe test.

Thanks mate, that makes sense.

 

[Triple_Se7en]

Nothing wrong with the Castrol, I've ran it a couple times for cheap oil changes when towing in my Expedition.

Got that right!
My Kia loves 75% Castrol Edge Euro 5w40 with 25% Castrol Edge EP 5w20.
Had noisy Quaker State Euro - then noisy Amsoil Euro, the two OCIs prior.