Technical query about viscosity

[FTO-UK]

In considering SF's MTL-P and SynchroGlide for my manual transmission I noted that the SynchroGlide has a much higher Viscosity Index than the MTL-P. This means that its viscosity changes less with temperature and consequently it is much less viscous than MTL-P at low temperatures.

Now, most of the people who post on here have problems with shift quality at low temps - usually due to the oil being too thick I guess. That got me to thinking - why aren't all gear oils formulated to have a high VI? Surely the 'Holy Grail' of a good gear oil would be one that is of a constant KV with respect to temperature? That way the lubricating properties and shift behaviour wouldn't change as the vehicle warms up and we wouldn't be struglling with baulky gearshifting on a cold morning.

Is there a downside to having a high VI in a gear oil? I can imagine the VI-improving chemicals might deteriorate with time, but surely with modern synthetics that can be overcome?

 

[y_p_w]

I've heard it described this way: In general, API GL-5 gear oil is a commodity product. There probably aren't that many manufacturers of it, and most of the sellers want a cheap, reliable, and compatible product. Of course there are some people who are willing to pay a premium for a more unique product, but we're in the minority.

Ordinary GL-5 gear oil does a decent job and I'd guess will satisfy most of the people who drive manual transmissions. It works reasonably well at warm temperatures. Of course I like to tinker around with my car's fluids and have settled on Motul Gear 300 75W90.

 

[MolaKule]

The VI is a direct function of the base oil and the VII if used.

Our oils have high VI because our base oils have a high VI.

There is nothing wrong or disadvantangeous about having a high VI GL since it means the oil is more stable, viscosity wise, over wider temperature ranges.

 

[FTO-UK]

quote:

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Originally posted by MolaKule:
The VI is a direct function of the base oil and the VII if used.

Our oils have high VI because our base oils have a high VI.

There is nothing wrong or disadvantangeous about having a high VI GL since it means the oil is more stable, viscosity wise, over wider temperature ranges.

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OK, I understand that. But the VI of your SynchroGlide is about double that of your MTL-P. Why not make an MTL-P oil with a higher VI (and hence thinner when cold)? Surely that's what we all want isn't it? Its akin to having a 0W-40 engine oil instead of a 15W-40 - it would mean better lubrication and easier gearshifting when cold.

 

[mechtech]

FTO - It seems tht Synchroglide is what you want.
Much thinner when cold/cool/warm, and only slightly thinner when hot than MTL P.

 

[FTO-UK]

quote:

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Originally posted by mechtech:
FTO - It seems tht Synchroglide is what you want.
Much thinner when cold/cool/warm, and only slightly thinner when hot than MTL P.

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Yes, I'm going to try SynchroGlide. However my original query on this thread is more general (I'm trying to educate myself a bit!). I'm wondering why ALL modern gear oils aren't made to have a high VI - its obviously easily possible (e.g. SynchroGlide). Why do we still have gear oils that get so thick at low temps?