My 97 K2500 had a G80, and it was a full locker. It worked perfectly too, on 80W90 no additive. The G80 works like an open diff, until a significant difference in wheel speed occurs. The actual difference has been rumored to be anyway from 10 RPM to 100 MPH, either way, it doesn't matter. Once this difference is reached, a flyweight moves and instantly locks both axle shafts together. At this point you have a full locker, power is transmitted equally to both sides. On paper and in theory it's a perfect setup. No effect on street handling, but a full locker when you need it. In practice, however, there are some issues. The primary problem being the sudden lock and transfer of power. This can create quite a shock load on the carrier, often resulting in a broken carrier at the passenger side of the bearing. GM further magnified this weakness when they "upgraded" the 10 bolt from a 28 spline axle shaft to a larger diameter 31 spline shaft. Rather than redesign the carrier to maintain strength at the bearing area, which would have required modifying the carrier bearing and possibly the housing, they simply took more meat out of the carrier.
Regardless, a quality synthetic gear lube like Mobil 1 is all you need.