I posted a few days ago with the same question for my 2007 3.0 x250, I got the same response: go for GL4, not GL5. There are some links to articles about this in my thread:
https://www.fiatforum.com/ducato/489119-gearbox-oil-x250-3-0-euro-4-a.html
Hi nigelvan. I enormously enjoyed reading the link you posted here. I enjoy anything to do with cars and look after our family vehicles myself. Having worked for Firestone early on in my career I have a particular fascination with tyres but I also find lubricants of particular interest and I'm always up for expanding my awareness.
The linked article describes most excellently the problem with the GL4 vs GL5 "thing". It's a problem surrounded by ignorance with many thinking that GL5 is a "better" oil to use because it's got more anti friction ability. Indeed I've heard it described as such in retail outlets. On the other hand I've heard more informed mechanics saying not to use GL5 where GL4 is specified because it "dissolves" the synchros! Nice to read here that the metal is removed, at the molecular level, by the bonding action, and subsequent tearing away, of the sacrificial anti friction additive bonded to the yellow metals which removes a tiny amount of yellow metal every time the mating surfaces pull apart - Read the article if you're more interested.
From what I've read the use of GL GL5 in situations where GL4 should have been used has caused enough concern amongst the manufacturers that most have now modified their formulations so that this is nothing like the problem it initially was - and there are now oils rated as GL4/GL5 so supposedly suitable for use where either is specified? Interestingly the Tutela Technyx 75w85 specified for our Panda and Punto is quoted as a GL4+ (The API don't even quote GL4+ but my guess would be that it's a GL4/GL5 spec'd oil which they've called GL4+ so as not to "spook" the horses?)
Personally I have to say that unless a GL5 oil is actually specified by the manufacturer I would still tend to default to a GL4 by preference for gearboxes in general. Having said that though I would always buy the actual gear oil to the actual specification recommended by the manufacturer. Interesting that the article particularly mentions Honda gear oils. I'm friendly with our local Honda indy specialist and call in on him for a tea time chat from time to time. He is very very keen to never use anything other than an actual Honda gearbox oil - this article seems to lend some credence to his advice?
At the end of the day you don't change gearbox oil very often and there's generally not a large amount involved - even less for topping up - so, although it might be relatively expensive buying the manufacturer specified product, for me it's got to be the genuine article every time, especially when you learn that some boxes don't even use a "traditional" gear oil but an automatic 'box oil or even a thin engine oil - rare, but I've seen it, after all the old "unit" type BMC stuff ran engine oil lubed gearboxes and I seem to remember some of the RWD Rovers used engine oil in their 'boxes, didn't they?
This may interest you:
https://www.widman.biz/uploads/Transaxle_oil.pdf Page three has an interesting bit for us about the API GL4+spec'd oils and their uses.
