Difference in viscosity between 75 and 80? MASSIVE! I had 80w90 in my car and it was impossible to shift on a cold morning. Now with 75w90 its much much better. The specs show the same story:
75w90:
Viscosity at 40°C in mm2/s: 88
80w90:
Viscosity at 40°C in mm2/s: 159
As you can see the 75 is better suited for low temperatures. This is a comparison between 2 oils of the same manufacturer.
GL5 oil is more agressive to some parts of the gearbox especially the brass parts! Avoid!
Yo guy at the counter knows nothing i guess