The GL4/GL5 thing that seems to be getting more and more well known is based on out of date information. Back in the 1970s, when GL5 oils were first introduced, it was very quickly realised that the additives attacked yellow metals. That was no good to the oil manufacturer or user, so the additive pack was soon changed (by reputable oil companies) so that they use the same additives as in a GL4 oil, but in a higher concentration. That means that GL5 oils from reputable companies will not attack brass synchros. I don't know if that is the case with oils from companies that we don't deal with though.
There are certain GL5 oils that are better as diff oils as they are too slippery for synchros, but they are not attacking the brass.
The Castrol one you are looking at is a bit thick, it should be a 75w-80 and the best proven options are the Red Line MTL, Amsoil MTF or Fuchs Sintofluid. The Millers TRX 75w-80 and Motul Motylgear are good alternatives.
http://www.opieoils.co.uk/c-670-75w-80-75w-85-mtfs.aspx
Cheers
Tim
