Why are these 2 oils different?

[kilou]

Hi,

I'm really worried by gearoil specification requirements in some european vehicle applications. Look at these 2 oils made by Elf for manual transmissions (sorry in french, cannot find it in english):

TransElf NFJ:http://www.lubadmin.com/upload/produit/FichePDF/lang_1/7194.pdf
TransElf NFP:http://www.lubadmin.com/upload/produit/FichePDF/lang_1/7195.pdf

Both oils are 75W80 API-GL4+ (which means that they are GL5 safe for use in GL4 applications). They are suitable for brass syncronizer rings and use specific additive packs that are non corrosive to yellow metals.

AFAIK the NFJ is a semi-synthetic product while the NFP is a higher quality fully synthetic product. Therefore I do understand that gearboxes calling for the NFP cannot use the NFJ instead but in my understanding it should be possible to use the NFP in a NFJ application since the NFP is a superior product that covers the same specifications. However the NFP is "not recommended" in the applications where the NFJ is recommended. Why is this the case? Is there anything beyond grade and API rating that would make a gearoil unsuitable for some applications?

Thanks

 

[Ursae_Majoris]

There is an ongoing discussion on the Toyota forums about dino VS Synthetic oils in Tacoma LSD rear differentials. Apparently the clutch pack material used by Toyota does not like synthetic gear oil. Tacoma differentials would make noise even with multiple bottles of LSD modifiers added.

So, here is one example.
http://www.toyotanation.com/forum/showthread.php?t=196715