Lowest Viscosity Index 10w30
- Thread starter thickvthin
- Start date
Castrol GTX 10W-30 synthetic blend - VI 139
API SP, ILSAC GF-6https://msdspds.castrol.com/bpglis/...7F7FD4D80258B2E00379496/$File/wepp-d5vk23.pdf
I still cannot understand how conventional and synth blend oils cover the same API and ILSAC specs as full synthetic oils.
Castrol
What are you interested in?
Patman
Staff member
Such an ignorant statement
- Joined
- Jan 18, 2026
- Messages
- 95
VI for readily available synthetic 10w30 oils at walmart, autozone etc. See if anything interesting pops up. I don't have anything particular in mind.
If it's a "full synthetic" then that places a floor under just how cheap they can go on base oils. Which means a 10w-30 is likely to have a bit less VII than a 0w would.
Because VII compounds are used a low treat rates and are fairly inexpensive, a normal blend would add VII to any base oil, taking the opportunity to cheapen base oils rather than lower VII treat rate. Base oils are a larger blend fraction and a bigger cost savings than reducing VII.
But constraining to group III or better means you will have at least a VI in the base oil of 120 or better. Which means a 10w-30 with a finished VI of 140-150 or so will have some VII in the formulation, but perhaps at a lower treat rate than a 5w with a higher required VI (155+).
Just remember that golden rule of formulation for shelf oils: the cheapest path to meet the spec is the path that is chosen. So, outside of HPL and several boutiques, the base oils are only as good as they have to be and no better, with low cost being the target as shelf oils are commodities. (as evidenced by the many users here trying to save <~$50 a year on oil changes).
It's not going to.
Every "full synthetic" that can be made with a low cost group III is made with low cost group III. Shell/Pennzoil uses a lot of GTL because they are heavily invested in it, but you'll find even someone has heavily invested as they still doesn't use 100% GTL in their bases (within solvency limits), it's mixed with regular group III in significant amounts.
Unfortunately for shelf oils, there's simply no way to trade VI for base oil quality. If you lower VI, they just cheapen the bases instead of cut the VII content.
We've covered this extensively in my discussion of HPL monograde SAE 40.
Last edited:
Patman
Staff member
And this is the reason why I say that 10w-30 is mostly an obsolete viscosity nowadays, because the majority of the ones that are out there are not as good quality as their 5w-30 and 0w-30 counterparts. Amsoil and HPL seem to be the exception to this rule mind you.
But anyone who thinks that they’re getting a more shear stable oil with 10w-30 over 5w-30 and 0w-30 needs to rethink that if they’re buying oil from the likes of Walmart, AAP, etc
It was hard for me to accept at first, also. I wasn't cynical enough, or rather I just didn't appreciate how cutthroat the margins are on commodity products like motor oil and even 50c cost savings on a quart is huge.
Old oil myths die hard. If my monograde HPL thread doesn't prove that, I'm not sure what does!
- Joined
- Jan 18, 2026
- Messages
- 95
sometimes people look at data and find things they had.not thought of before. It was not part of their hypothesis.
May be true, but the price is right.
But what was your hypothesis? That's what most of us were trying to find out.
And yet, that’s your preferred oil, is it not?
What’s in your stash -
Because the requirements are based on performance, not method of manufacture. As long as the end result is the same there may not a need for a hydrocracked base stock, or at least not as much. Other methods of manufacture are catching up.
I understand but some of those tests include high temp. performance, cleanliness, timing chain wear, etc.
What would be the advantage of full synthetic/hydro crack oils if the conventional and synth. blend oils cover all those same test?
Just the longer OCI of the full synthetics?
Higher VI, less contaminants, higher saturation which lead to requiring less VM for a given performance target. All of which leads to fewer deposits.
I’ve never seen “cleanliness” in API Annex E but no doubt it appears in some form in many licenses or approvals. Hydrocracked Group III will always be better in that regard due to the synthesis since you’re never able to remove some contaminates otherwise. They from azeotropes or other conditions where contaminates cannot be “washed” out.
Base stock group designations are ranges for performance properties but a Group III will always be superior in those areas over a Group II.
Below was my go-to oil a few years ago when I owned a Ford CUV with a notorious fuel diluting 2.0L direct injection engine.
This was back when we could guess that the base was majority GTL. The guess was from consideration of the reported VI and PP and then the 'meets exceeds' API SP-RC and ILSAC GF-6A ... and being fully aware of the Shell Qatar Pearl GTL project.
How many affordable "Fully Synthetic" motor oils demonstrate a PP of -54 deg C (ASTM D97). I ran it until Shell decided this was too big of a "gift" to the common man and brought the product line back round to ho-hum performance.
Oh, yeah. You know old Arco likes to pull this dogeared folded paper out of his back pocket whenever he gets a chance.
- Joined
- Jan 18, 2026
- Messages
- 95
sorry, I did not have one.
No. I only use it as weed killer now.
I used to put Castrol in my turbo diesel, but since the new year I've been using Valvoline for my daily, zero mile OCI's.
Last edited: