Why do you say that Baz? What is in a datto box (and apparently not in any other box) that is going to be effected by a slightly different oil? To my knowledge the only thing that stuffs synchros is bad gear changes or too much torque!
Synchros are designed to wear and have a limited life span anyway. They are supposed to be replaced. Hence, a gearbox is a maintainable item. The properties of a combined diff/gearbox oil should not effect the performance of a synchro any more than a specific gearbox oil. The performance of any given oil has everything to do with the pressure (molecular structure strength), viscosity and temperature properties of the individual oil.
Scott, I don't believe a universal diff/gearbox oil will stuff the synchro's so to speak, but I do think that putting in an oil specifically designed for the job is a good idea. Hence, you should put in a manual trans oil, unless it is a competition gearbox. Which is another story.
:shock:Ok actually it dosnt stuff the synchro just stops them doing the job, which i guess is much the same thing. I havent a copy of the Nissan service bulletin warning about the use of other than manual Transoil.
I am of the understanding that the, hope i have the right name here, "Thio-hypoid"? additive that is used in diff oil, to prevent scuffing of the crown wheel & pinion, is present in universal trans/diff oil. This additive plates the synchro rings and reduces their braking action. Many, times i have seen Nissans, mainly with warner type, that have crunching synchros cured just by changing the trans oil to the correct type. It takes about a week or 2 for the box to come good again.
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"R.I.P. Baz. 29 April 2022. Thank you for all your contributions to the Datsun community over the years. You will be missed." - OZDAT