Jock,
You are spot on, if the gear oil is too slippy it can lead to crunching during gearchanges especially at high rpm.
The additive used that damged yellow metals was abandoned by all reputable oil companies the UK during the mid 1960's (yes, how long the yellow metal myth has been going on!).
Often what happens, is say a high quality GL5 is used in an application that is much happier on GL4 and the gears start to crunch. The first thought after looking on the internet is yellow metals are getting eaten by the GL5, the gearbox gets drained and bits of yellow metal are found and then it is confirmed the GL5 is eating the yellow metal, when in fact that is not the case at all. The yellow metal comes from wear of the synchros due to the poor shift of GL5 being too slippy for that application.
GL4 and GL5 these days use the same yellow metal safe EP additives just in different amounts.
Hope this helps!
Cheers,
Guy