I use Mobil1 full synth - it IS a GL-5, and you won't see anything about yellow metals on the label, however, a visit to their website should show something along the lines of safe for use with copper & it's alloys.
Of course - my earlier warning still stands - if YOU don't see anything about yellow metal friendly etc - use at YOUR own risk - I've been using it in the transmission of all my Suzukis for many years so far I'm happy with it - unfortunately - the Exxon-Mobil distribution network here was bought out and it's no longer available, so next time around I will have to use something else.