There are many rear diffs out there. The easiest way to ID then is by the code tag as it contains all the information you need. Manual rear diff ratio is always 3.545 and automatic is always 3.307. The only difference is the 3000GT VR4 and Stealth RT Twin Turbo. 3.307 is a 6 speed manual diff but its the same ratio as Auto so it can be used with no issue.

If your code tag on the rear diff cover isnt legible, missing, or you have an aftermarket diff cover for the rear differential then you can put axles in the diff. Lock one of the axles to prevent the axle from rotating. Put and axle in the other side and spin the rear diff input shaft 10 times. If the free axle rotates 33 times then its automatic. If it spins 35 times then its a manual. My rear diff the id tag didnt exist so I sat the rear differential on a workbench and locked the axle in a vise and setup a camera to watch. I marked the input shaft and the free axle. so you can see when each one makes a rotation. I spun the input shaft and focused on spinning it 10 times. I then went back and looked at the video and confirmed 10 rotations. Then i counted the free axle rotations and it made 33 full rotations. Successfully identified as a automatic rear diff.

Here is a table with the rear diff codes. As you can see from the pattern. If the tag code starts with J then it is the automatic 3.307 ratio and tag code starts with B then it is the manual 3.545 ratio.

The automatic rear differential fluid capacity is the same as the manual transmission fluid capacity.

If your wondering what the best fluid is for your rear differential then please see the link below to Team Rip Engineering. He outlines this well. Team Rip Engineering REAR AXLE OIL

The 3000GT VR4 / Stealth RT Twin Turbo differentials are alot stronger then the DSM rear differentials. As a result of its robustness the unit is indeed heavier. Also one thing to note is the DSM Axles do not work with one of these differentials. You will have to either have stock 4 bolt axles rebuilt with 3000GT inner cups. Driveshaft shop can also do the same on their axles with an extra charge. Below is a listing of these diff codes so you can identify them. For automatic you want the 6 speed differentials since they are 3.307 ratio which is the same as the DSM auto rear differential ratio. To the left you can see a DSM 4 bolt rear differential next to a 6 speed 3000GT VR4 / Stealth RT Twin Turbo rear differential. To blockoff the rear power steering pump that come in these since they are not needed just get a freeze plug and some RTV on the side and tap it in with a socket until the edges are flush.