Gear box oil

[rick]

I've never heard of Multigrade Transmission Fluid either, but that doesn't mean someone doesn't use that term, just one I'm not familiar with.

Only ever seen Manual Transmission Fluid used, including by Castrol Oz who list Syntrans of various flavours and Syntrax Universal as Manual Transmission Fluids.

And just to upset the apple cart, I love Motul Gear 300 in the R380 which is a (gasp !) 75W-90 !

Shifts fine at -5*C, and no bearing rattle at 35+*C ambients which you even get with decent MTF's such as Castrol Syntrans 75W-85.

You do have to be quick with the 1st-2nd change initially, a little hesitant and it'l baulk or worse, crunch, but within a few shifts it works fine.

ATF is fine at 0-20*C ambient and no heat in the oil, but is too bloody thin in Australia, shifting goes to hell quickly and the gearbox sounds like a bucket of bolts with any decent temp in it, and FWIW I've recorded 75*C and the temp was still climbing within 7km of a highway drive.

Apart from the fact ATF has the wrong friction modifiers for the synchro's to engage properly, and the additive package is weaker than any GL4 MTF so bearings and gears aren't anywhere near as well protected as with any decent MTF of whatever viscosity you choose to run.

Kinematic viscosity does have a big bearing on shiftability when cold (and hot), (the other factor is the additive package, including the level of EP additives, as well as the specific friction modifiers used for synchro hub/clutch engagement) but I'd rather have decent changes when warm and under load and suffer a few potential missed shifts, if indeed they do happen, on a cold morning.

[tweetyduck]

Can't be arsed to read it all. Redline MTL is ~75/90...MTF 94 is pretty much ~75/90

(BTW there are AT LEAST two different types of Redline MTL) I think i have bottles for 75/90 75/80 and 70/80 IIRC

just put in MTF94 and forget about this. We are lucky we can get it. I had a nightmare in the USA.

[Boydie]

There are three types of transmission fluid, Automatic, Manual and Multigrade.

The Automatic is quite obvious in that it is designed for automatic gear boxes has LOW friction properties as it has to allow for friction clutches and, due to the small orifices within the valve plate, a very low viscosity, typically around 5W.

The second type of transmission fluid is designated Manual, this is normally a semi or full synthetic fluid, again with a fixed viscosity, again around 5W.

The third is a fully synthetic Multigrade fluid which has a low viscosity at both ends of the gearbox normal operating temperature spectrum, it has a low viscosity when cold and as the fluid temperature increases it's metabolic structure changes to become thicker so it actually maintains the original low viscosity.

Both the automatic and manual fluids being a fixed cold temperature viscosity become thinner as their operating temperature increases and therefore have an even lower viscosity as they become hotter.

All three, being transmission fluids are low friction.

The advantage of the more expensive Multigrade is that it is designed to have a constant viscosity at both ends of the operating temperature spectrum.

The R380 gearbox oil circulation pump is the reason for the original specification of ATF as it is unable to pump higher (thicker) viscous oils, it's a very basic rotary pump and the use of thicker oils could result in either the mechanical failure of the pump or the metallic-asbestos seal could blow resulting in zero oil flow through the shaft and the shaft bearings. The other stymie is the fine gauze filter on the pump inlet, it's far too fine for thicker fluids.

Living in Australia where we have a huge variety of ambient temperatures - daily - consequently I use the Multigrade, however were I to live in the colder Southern Alps down in Victoria and not touring the hotter central Australian Deserts I would opt for the ATF or Manual transmission fluid due to it's lower cost and lower viscosity.

[rick]

There are three types of transmission fluid, Automatic, Manual and Multigrade.

The Automatic is quite obvious in that it is designed for automatic gear boxes has LOW friction properties as it has to allow for friction clutches and, due to the small orifices within the valve plate, a very low viscosity, typically around 5W.

The second type of transmission fluid is designated Manual, this is normally a semi or full synthetic fluid, again with a fixed viscosity, again around 5W.

The third is a fully synthetic Multigrade fluid which has a low viscosity at both ends of the gearbox normal operating temperature spectrum, it has a low viscosity when cold and as the fluid temperature increases it's metabolic structure changes to become thicker so it actually maintains the original low viscosity.

Both the automatic and manual fluids being a fixed cold temperature viscosity become thinner as their operating temperature increases and therefore have an even lower viscosity as they become hotter.

All three, being transmission fluids are low friction.

The advantage of the more expensive Multigrade is that it is designed to have a constant viscosity at both ends of the operating temperature spectrum.

[snip]

Err, no, sorry, that's not how these fluids function at all, and your nomenclature is wrong too.

Firstly, MTF = Manual Trans Fluid, whether it's a 75W, or 75W-80, etc.

That's how the oil companies describe them, check out the PDS from any major.

'Conventional' mineral, semi-synth or synthetic fluids, be they ATF's, Manual Trans fluids (and that includes xW-70/75/80/85/90's) and gear/diff oils are Newtonian in nature regardless of whether they are straight grades or multigrades. ie, as the fluid temp increases their kinematic viscosity decreases.

Multigrades exhibit a wider operating temperature range over a straight grade as they thin less with increasing temperature increase.

That's the only magic. Even multigrades are thicker at 0*C than they are at 40*C and 100*C, much thicker.

The rate of viscosity thinning is usually moderated by viscosity index improvers, specialised polymers that reduce the rate of thinning.

Some high end synthetics have little to none VII's used, but they are the exception, not the rule.

Multigrade oils used in gearboxes, diffs or engines cannot get thicker or even maintain their viscosity as they get hotter ! They get thinner !

It's physics !

eg Redlines MTL is a 75W-80 Manual Trans Fluid.

Its kinematic viscosities are 54.1 cSt @ 40*C and 10.4 cSt @ 100*C.

its viscosity index (VI) is 185.

Plug those numbers into a viscosity index calculator and its viscosity @ 0*C is 554 cSt.

It's become thinner as it gets hotter !

It's just that the rate of thinning is less than if it was a straight grade SAE 80 mineral lube.

If it was an SAE 75W it may be thinner than an SAE 80 @ 100*C or it may meet the SAE 80 viscosity range. eg. Castrol Australia's VMX-80 manual trans fluid is described as an SAE 80W but does in fact meet the SAE 75W-80 viscosity range as it's hot viscosity is 10.5cSt @ 100*C, smack bang in the SAE 80 gear (and SAE 30 engine) oil range.

The only lubricants that are non-Newtonian in nature are specialised silicone based fluids such as used in viscous couplings which increase their kinematic viscosity when sheared (and heated)

I wrote some stuff years ago on base oils, kinematic viscosity, viscosity index improvers (the polymeric thickeners used to reduce the rate of oil thinning as temp increases) here http://forums.lr4x4.com/index.php?showtopic=27742&page=2#entry277688 and here http://forums.lr4x4.com/index.php?showtopic=27742&page=2#entry277916 and while discussing engine oils, the viscosity characteristics exhibited by ATF's, MTF's and gear oils are the same.