ZBUM's Nissan Z31 300ZX Differential Information Page
All Z31s came with the R200 differential. Keep in mind that during crossover months (September) models may have either the listed ratio or the year prior.
Differential Fluid Levels:
Fluid Temperature Ratings:
Differential swaps are common and very simple. Swapping any non viscous LSD into your open differential car only requires the differential. Depending on what year your driveshaft and differential are, you may have to swap companion flanges. There are only two differential companion flanges. The 84-86 models have a slightly offset bolt pattern (i.e. make a rectangle). The 87-89 models have a equal bolt pattern (i.e. make a square). Swapping companion flanges usually requires an impact gun.
Swapping in the viscous LSD from an 88SS model is a bit more intensive. For all models, you will also need the half shafts. For 84-86 models, you will also need to swap the driveshaft companion flange. For non-turbo models, you will also need to swap the outer hubs in the control arms for turbo hubs.
These differentials are indestructable, change the oil every year or two and you'll never have a problem. Nissan Motorsports sells complete ready to install R200 clutch LSDs for approximately $700-$1100 with your choice of standard R200 ratios.
Phantom grip r200 installation procedure is here. Note: According to some owners, Phantom Grip sucks. Their recommendation is to splurge for an LSD.
To tell if you have a differential, lift both rear tires off the ground and turn 1 wheel. If the opposite wheel turns the same direction then you have a LSD. If not, you have an open diff. The transmission should be in neutral for this.