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The mkiv Supra Owners Club

Cold Weather Gear Changes


oilman
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Hi all,

 

Around this time of year we get quite a few questions about experiencing difficult gearshifts in cold weather, so we thought you might find this useful.

Using the incorrect viscosity gear oil in your car can make gear changes very difficult when the gearbox, and oil are cold. This is why we often don't recommend using thicker gear oils than the manufacturer specifies. It is easy to think that increasing the viscosity from a 75w-80 to 75w-90 or 75w-90 to 75w-140 isn't going to affect the cold properties of the oil, but it does.

Below are some figures showing the viscosity of a selection of oils at 40C measured in mm²/s ( 1 millimeter²/second = 1 centistoke, A centistoke is a decimal fraction of the CGS unit of kinematic viscosity stokes, which is equal to centimeter per second (cm²/s). 1 stokes is a kinematic viscosity of a fluid with a density of 1 g/cm³ and a dynamic viscosity of 1 poise… In short the thickness of oil!). 40degc may not seem cold but this is the temperature at which the viscosity is measured and is the information you will find on the oil technical data sheets.

I have listed specific brands and products to make it easier to see the differences in viscosity.

Fuchs Sintofluid FE 75w = 40.8 @ 40°c

Fuchs Sintofluid 75w-80 = 49.8 @ 40°c

Fuchs Sintopoid 75w-90 = 90.5 @ 40°c

Fuchs Sintopoid LS 75w140 = 170 @ 40°c

 

As you can see from that, they are all 75w oils, but there is a large difference in the viscosity at lower temps; the Sintopoid 75w-90 is over twice as thick as the Sintofluid FE 75w when cold and the 75w-140 is almost twice as thick as the 75w-90.^

 

So, if you've got a car that needs one grade of oil as standard, but you've modified it and decide to try a thicker oil, or someone puts the wrong oil in, it might really affect the cold gear shifts. Usually, increasing the quality is a better option than increasing the viscosity when thinking about upgrading from standard fluid.

 

Also, viscosity ratings are not exact points, but are a band that the viscosity should fall in. The Motul Gear 300 75w-90 has a viscosity of 76.2mm²/s at 40°c and the Castrol Universal 75w-90 is 84.8mm²/s, so you can see there is some variation in oils that appear to be the same viscosity.

 

A lot of gearboxes specify an 80w-90 rather than a 75w-90, but I would always tend to go for a 75w-90 instead as there can be a large difference is the viscosity when cold. The Motul Gearbox 80w-90 is 164mm²/s, so over twice as thick as their Gear 300 when cold. If you're using an 80w-90 and are having stiff cold gear changes, changing to a 75w-90 is likely to improve things.

 

If you have any questions or need further advice please post here or email us at [email protected]

 

Cheers

Tim.

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Hi Tim,

Thats some great info.....but, its fine if we know the oem spec oil viscosity like in the w58 box we can use a "better" alternative.The problem comes when we refer to the now discontinued 6 speed oil v160, does anyone or has anyone actually tested this oil for grade, modifiers etc, i was always led to believe it was more of a atf fluid so actually an engine oil would be more suitable of 10/40?(not that i`ll be trying it). please don`t tell us r/p syncro whatever is the alternative as ive been assured its not.

I did alot of reading up about gearbox oil for my van(mtx-75) recently and it turns out this requires "special" oil not any old 75/90, i may get the ford honey that`s recommended for it @ £16.50 per litre actually xt-m5-qs i believe.

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Hi

 

The V160 oils is an ATF and we have a few that are suitable to use as an alternative.

 

http://www.opieoils.co.uk/c-1432-atfs-automatic-transmission-fluid.aspx

 

The ones to go for are the Amsoil ATF, Millers SPIII, Motul Multi ATF and Fuchs ATF4400. I think the most common choice is the Amsoil ATF.

 

I would not recommend the RP stuff, it's not something we sell or something we want to sell. I've spoken to plenty of people that have been recommended that for a lot of cars that need an ATF in the gearbox and they have said the changes are too stiff. Perhaps it's okay when an MTF is needed, but not in place of an ATF.

 

Cheers

 

Tim

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Very interesting (if uber-technical... :conf: ) first post. It explains a lot - I've been struggling with really stiff gear changes (V160 'box) over this winter until everything gets nice and warm and I think this answers the question of why. I'll certainly be trying some of the ATF fluid you recommend as soon as I get some time to do it.

 

Thanks Tim! :)

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