Thread: Isuzu gearbox oil

Originally Posted by Dougal

I'm using Castrol VMX80. Rick suggested it.

Yeah that's what I'm also using sorry I said apx before ...now edited

Originally Posted by Bush65

Any synthetic manual transmission fluid. I use Castrol Syntrans when it is available.

BTW never use the term 'gear oil'in place of manual transmission fluid. You need to re-program your brain. Gear oil is not suitable for manual gearboxes in automotive use.

Because gear oil was not suitable, and in the early years nothing better was available, manufacturers specified engine oil for their gearboxes. In later years some changed to automatic transmission fluid.

Oil manufactures have since developed manual transmission fluid (MTF). It has the low viscocity and high viscocity index needed, with additives for the extreme pressure that occurs between mating gear teeth. It is far superior to engine oil and auto transmission fluid.

Lots of people here will tell you engine oil and use it in their LT95 gearboxes. I can't say here that I think they are ...... either that or they are ignorant of the differences between engine oil and MTF and still think MTF is gear oil (gear oil is too viscous and will kill an LT95 after the oil pump gear fails).

Sorry for the use of words but I admit I have got no idea what so ever about oils so I'm listening intently...

Which Syntrans do you use John? I know they make several, I'm using VMX80 because Rick130 suggested it based on the viscosity match.

I'm wondering if something a little thicker than VMX80 might have some noise reduction properties.

FWIW, my original MSA-5P box specified 15w40 engine oil. My current MSA-5G box specifies 5w30. My MSA-5P didn't exhibit the low rpm high torque rumble that my 5G does. It wasn't taking the same torque, but it wasn't that far off.

John,

I am after some clarification about the engine oil vs MTF questions you have raised.

In my limited opinion ( Compared to others more informed about oil choices on this site) I would think that when LR designed the LT95 and specified engine oils, the oils on offer back then were 'suitable' for an archaic design such as this transmission. Modern 15w40 oils are far better surely?

True the torque us Isuzu owners push through these gearboxes will well exceed the little 3.5 V8 so, maybe that is good enough reason to change to an MTF?

However I have been using the 15w40 for 150K now, in both box and transfer, ( Taper rollers for 100K after the thrusts almost caught fire ) and to date no noticeable wear and certainly no failures have occurred.

HOWEVER;
I am always open to trying a better product, if it wasn't for the price (6 litres @ $30/l !) I would be using the Syntrans 75w85 option, for sure.

JC

Originally Posted by justinc

John,

I am after some clarification about the engine oil vs MTF questions you have raised.

In my limited opinion ( Compared to others more informed about oil choices on this site) I would think that when LR designed the LT95 and specified engine oils, the oils on offer back then were 'suitable' for an archaic design such as this transmission. Modern 15w40 oils are far better surely?

True the torque us Isuzu owners push through these gearboxes will well exceed the little 3.5 V8 so, maybe that is good enough reason to change to an MTF?

However I have been using the 15w40 for 150K now, in both box and transfer, ( Taper rollers for 100K after the thrusts almost caught fire ) and to date no noticeable wear and certainly no failures have occurred.

HOWEVER;
I am always open to trying a better product, if it wasn't for the price (6 litres @ $30/l !) I would be using the Syntrans 75w85 option, for sure.

JC

JC,

I think that there is one other factor that will come into it, how often you change the oil? MTF will be more resistant to breaking down due to the chopping of the gears, so should last longer than engine oil.

But I am not rick130 of named John.

Originally Posted by slug_burner

JC,

I think that there is one other factor that will come into it, how often you change the oil? MTF will be more resistant to breaking down due to the chopping of the gears, so should last longer than engine oil.

But I am not rick130 of named John.

Good point, I change mine at every engine oil change , 10,000km, because I can (I buy engine oil in 205litre drums...). Since this subject is raised again, I will give a designated MTF like penrite SIN75 a try next change. (Due now actually...)

JC

Originally Posted by justinc

John,

I am after some clarification about the engine oil vs MTF questions you have raised.

In my limited opinion ( Compared to others more informed about oil choices on this site) I would think that when LR designed the LT95 and specified engine oils, the oils on offer back then were 'suitable' for an archaic design such as this transmission. Modern 15w40 oils are far better surely?

True the torque us Isuzu owners push through these gearboxes will well exceed the little 3.5 V8 so, maybe that is good enough reason to change to an MTF?

However I have been using the 15w40 for 150K now, in both box and transfer, ( Taper rollers for 100K after the thrusts almost caught fire ) and to date no noticeable wear and certainly no failures have occurred.

HOWEVER;
I am always open to trying a better product, if it wasn't for the price (6 litres @ $30/l !) I would be using the Syntrans 75w85 option, for sure.

JC

JC, MTF's are much more resistant to shearing than engine oils, and generally the friction modifiers that are used are much more suitable for synchro's (actually, they are used to enhance synchro performance )
The shearing problem isn't an issue with the frequency you change your geabox oil

In a nutshell, as the sycnchro hub and clutch get close to the same speed an engine oil may reduce its coefficient of friction and clashing (crunching) can occur.

A dedicated MTF uses friction modifiers that exhibit the opposite characteristic.

IMO VMX-80 is too light for an LT85 or 95, its viscosity is equivalent to a light 10W-30 engine oil.

VMX-M and Syntrans 75W-85 are equivalent to a heavy 10W-30 (Syntrans is around 11.9cSt @ 100*C)

A good 75W-90 MTF is equivalent to a 10/15W-40 engine oil.

Motul Gear 300 is pretty trick, at 0*C it's lighter than Syntrans 75W-85, yet at 100*C it's the same viscosity as a good xW-40 diesel engine oil.
If its dual rating GL4/5 bothers anyone as it's designed for transaxle and gearbox usage (and I think it's fine with yellow metals) Redline MT90 is a true GL-4 spec oil.
Syntrans M 75W-90 is probably too heavy at 18.something cSt @ 100*C for an LT g/box. (its primary application is Road Ranger 'boxes)

Of course there's more to shiftability than viscosity, as mentioned the friction modifers used, the rest of the additive package, etc. all play a roll.

Originally Posted by rick130

JC, MTF's are much more resistant to shearing than engine oils, and generally the friction modifiers that are used are much more suitable for synchro's (actually, they are used to enhance synchro performance )
The shearing problem isn't an issue with the frequency you change your geabox oil

In a nutshell, as the sycnchro hub and clutch get close to the same speed an engine oil may reduce its coefficient of friction and clashing (crunching) can occur.

A dedicated MTF uses friction modifiers that exhibit the opposite characteristic.

IMO VMX-80 is too light for an LT85 or 95, its viscosity is equivalent to a light 10W-30 engine oil.

VMX-M and Syntrans 75W-85 are equivalent to a heavy 10W-30 (Syntrans is around 11.9cSt @ 100*C)

A good 75W-90 MTF is equivalent to a 10/15W-40 engine oil.

Motul Gear 300 is pretty trick, at 0*C it's lighter than Syntrans 75W-85, yet at 100*C it's the same viscosity as a good xW-40 diesel engine oil.
If its dual rating GL4/5 bothers anyone as it's designed for transaxle and gearbox usage (and I think it's fine with yellow metals) Redline MT90 is a true GL-4 spec oil.
Syntrans M 75W-90 is probably too heavy at 18.something cSt @ 100*C for an LT g/box. (its primary application is Road Ranger 'boxes)

Of course there's more to shiftability than viscosity, as mentioned the friction modifers used, the rest of the additive package, etc. all play a roll.

Thanks Rick,

Gets a bit exxy using Sytrans to fill a Road Ranger

I might give the Valvoline Duragear 75w85 a try, Redline MT90 or the Penrite SIN75.

I'll report back then

JC