Google: GL-5 is not necessarily backward-compatible in synchro-mesh  transmissions which are designed for a GL-4 oil: GL-5 has a lower  coefficient of friction due to the higher concentration of EP additives  over GL-4, and thus synchros can not engage as effectively. Also,  transmissions which explicitly call for GL-4 oil may have been designed  around this lower concentration of EP additives and thus may contain  yellow metal parts which GL-5 will corrode.
interesting: 
http://www.widman.biz/uploads/Transaxle_oil.pdf
personally i'd play it safe and go GL4.