What gear lube do I use-Front and Rear?

[F-106 Delta Dart]

95 4WD XLT I want to change the oil in the front and rear axles. According to my owners manual it says to use "4x4 gear oil. That's fine but what IS 4x4 gear oil? In the rear axle it says to use 80W90-QL. For you people that have changed your gear oil what did you use? I want to go with a synthetic oil. In the rear do I need to use the friction modifier or will the synthetic be good enough? So what gear oil goes in front and what goes in the rear? Many,many thanks!