The dreaded manual gearbox noise...

[mydmax]

The point I was making is this,
A 5w oil is 5W when cold, a 30 W is 30W and cushioning gearbox parts far better. A 5W 30W will thin or claimed to not thin as much as a straight 5W oil would do, and you hope the 30W aspect is retained to some degree when hot. The 5W which thins anyway, only thins to the viscosity which W30 would be at that hot temperature. Probably that is thinner than a 5W. ie HOT gearbox tooth chatter noticed in neutral or low gears at idle.
So, if the transmission is not getting to the same or near engine temps, starting off with 5W means the gearbox is running mostly on a far thinner viscosity than intended by the maker of the gearbox.
There is no way I would run my Isuzu gearbox on 5W oil. Many people run thicker than 30W.
If 5W is used the idle speed gear teeth chatter would be very very noisey and gearbox relying on a high strength oil film between synchros and dogs and gear tooth faces will be deprived of that insurance level.

[jjliu0125]

Thanks Mike C fornyour replies. I will look into the Amsoil product. Thanks mydmax for your replies. What weight oil are you recommending or using? The whirling buzz noise I am hearing is only when in gear not in neutral idling. I am rereading these past post that are saying that the noise is from the 5th gear synchro. After swapping fluids, the noise was apparent but not as loud as it is after 2000km.

[mydmax]

jjliu
I use 30W but a good world wide brand name. I modified my 30W with a small amount of Mobil1 multigrade so it retains the cold shifting qualities and lubes when hot. Too thick will mean you have to change slower, ie, no racing changes. these are not racing gearboxes as some try to use them. Not having a later model I have not experienced the whining noise which many say is 5th synchro. Some have had the synchro replaced and have same noise.
Amsoil may be a good product but I don't believe Amsoil have tested 5w30 specifically in Isuzu gearboxes, only made a lube oil to a specification. So, unless they have done exhaustive tests on this gearbox, Isuzu has, an oil company simply provides a formula to a spec. How they can think 5W when cold is doing what a 30W when cold is providing is mystifying. To be any good Amsoil 5W30 has to be thicker at working temp than the straight 30W is at that same working temp.
Therefore,a good quality multigrade engine oil,ie 30W40 is starting at 30W and not thinning to less than 40W will at same working temp may suit. If not too thick when hot and causing change problems when cold, it will ensure greater protection of gearbox components. Remember 30W40 when hot and drained from an engine is like water, so we are not dealing with what we see when cold. Some experimentation may be needed to get what you want. Most oil viscosity offerings from a manufacturer can be mixed to achieve a slightly different viscosity to suit your needs.

I also add a little G70 Nulon to gearbox and transfer case oils and any non LSD diffs. Nulon is a high pressure surface lubricant when between gear teeth.

[jjliu0125]

Just wanted to give an update. I was not able to find Sae30 motor oil but did find 80w90 GL4 gear oil. Drained out the 5w30 motor oil and added the 80w90 and the buzzing noise is gone. Thanks Mydmax and MikeC for your replies.

[Mike C]

For what its worth, I'm currently in the country house where I keep the Dmax. I had a look in the Haynes manual for their recommended manual transmission oil and it's Dexron VI.

[mydmax]

Mike C
I love your faith in HAYNES manuals. A simple typo can make Dexron VI suitable for a manual box. What does the book for your vehicle say? Any difference there?

[Mike C]

I follow the book that came with the car, supplied by Isuzu. But interesting the driver's manual for the last of the Holden Colarado' s recommended using Dexron VI in the manual gearbox.

For anyone looking for an RG Colorado driver's manual, there's one here.

https://www.google.com/url?sa=t&rct=j&q ... SmN2_Zd4-5