Note that the 5-speed transmissions seem to be able to stomach more varieties of oil without complaint than the 6-speeds. The 6-speed transmission is a fussy beast. While the manual says that any GL-4 or GL-5 will work, that is because GL-4 fluids are very hard to find and a GL-5 will not hurt your transmission, however, “not hurt” is very different than “perform well”, and the GL-5 fluids will not perform well in your transmission.
The difference between GL-4 and GL-5 is that GL5 has twice as much Extreme Pressure (EP) additive in it. EP additive prevents the synchronizers from doing their job properly and results in hard or "notchy" shifting.
Even if you switch to a fluid with less EP in it, it may take in excess of 3 000 kilometers for the EP to wear off and the shifting to improve.