Anyone that starts a comment on a thread with:
"Hello to all, I work for an oil company so i know me stuff " - is really asking to be shot at big time, but I'm resisting the temptation for now.
What I was referring to is the fact that you quote viscocities at 40° and 100° as per the norm, but those figures seem to bear no relation to the product names ie EP 75-90?
As you will know I'm sure there are a vast range of EP 80-90 oils available and when looking at their viscocities as above, they range hugely, so i can't help wondering what is the point of the motor manufacturers recomending for example EP80-90 for a particular application when we know that the actual operating viscocity could be nearly anything !!!!!
IF they were to say an EP oil which has the characteristic viscocity of for example: 80 @ 40° & 25 @ 100° (figures plucked out of the air as per the TBL method of oil testing) then that would demonstrate a clear need for a specific oil. As it is, the particular application, (a transfer box for example) according to the vehicle manufacturer can run succesfully on just about any EP oil, so long as it is (in the case of LR) to GL5 spec.
This all leads me to conclude that it is not at all neccesary "to know your stuff" when it comes to oils, but just have a broad enough 'feel' for the subject to appreciate that it is not anywhere near as critical an issue as some would have us believe.
Changing oils regularly however does, imho, have a crucial impact on wear of assemblies.