This is going to challenge my English spelling, but here goes, making it as short/simple as I can because of not boring you but mostly because it is hard in English
:
Gear-oil is like engine-oil in the way that there will be a lot of different opinions to what to use. Lot's of experts everywhere
Fact: A gearoil's most important job is to prevent mechanical contact between the moving parts/gears. When the hypoid-gears came standard in gearboxes the need for some additives that could handle the high pressure between the gears was needed. The additives is called "EP" and stands for "Extreme Pressure".
The GL-4/5 labellings is telling how much of this additive(actually there is several) there is in the oil. The higher number, the higher pressure it can handle. So in theory, a GL-5 can handle more pressure than a GL-4, and if GL-5 is recommended you should not use GL-4.
You might think, why use GL-4 at all when the higher numbers have more "heavy duty" additives? The more "extrem pressure" it can handle the better, right?
That is a good question and here is where people are starting to have different opinions.
In most older gearboxes there is "yellow-metals", like copper and bronze. Used in synchromeshes, bearing-cages and so on. And the problem with the EP-additives is, that they are corrosive to these yellow metals. And the more additives/higher GL-number, the more aggressive the oil can be against these metals, and in the long run can cause synchromesh-failure, bearing problems etc. In newer cars this is no problem because the quality of parts have improved. In older cars, however, it is an issue.
I have done a lot of research about this regarding what gearoil to use in my Citroen DS from 1970. And I ended up using GL-4. Because at the time the DS was in production, GL-5 didn't exist, thus the GL-4 should be more than good enough. And a GL-4 is more "yellow metal"-safe. And because I already have GL-4 oils in my garage, I use it on my A-models too.
I dont think the old construction of the 2CV needs GL-5, and usually GL-4 is cheaper too!
Las time I changed gearoil on an A-modell I used this, EP80w-90 GL-4 :
(I am not advertising the brand, just showing typical spesification)
http://www.roadrunnerautoparts.co.uk/Product.aspx?id=6890Quote:
For synchromesh and constant mesh manual gearboxes.
Also suitable for use in hypoid, spiral bevel and double reduction rear axles.
Use in cars, vans, trucks and plant where an EP GL4 oil is required.
Quakerstate oils have a GL-4 where they specially mention it to be "yellow metal safe", so it is a issue even big company's think of.
I have used most Quakerstate-oils but only because a friend sells them to me really really cheap
All this said, I think it will be difficult to prove one way or another of damages from corrosive EP-additives, so it becomes a matter of believes I guess. Just wanted to mention it, and if people have knowledge about gear-oils and yellow metals, let's hear it, as I always like to learn more from other peoples opinions!
What is 100% sure, is what Sean says, a new oil will never be as harmful as an old dirty one!
And by the way, even if the gearoil is clean and looks new, it must be changed at intervals. Because the oil-molecules gets "crushed" between the gears in use so old oil looses it's function. Must be replaced, even if it looks "as new".
Daewoo 
'Fraid so .... 
