Aaron, the small car is almost square and the larger car is decidedly rectangular. Also, a 50% greater load rating on the bigger car.

My N5.8 came with both the long and short cars, with the small car installed. The PO apparently replaced the big car because it didn't feel very good. Well, after feeling how bad the "newer" short car was and then checking how the long car felt, I had to agree. Took the long car apart, cleaned the car bearing race with a small, round, nylon brush and soap and water, rubbed all the balls clean, cleaned the track, sprayed it all down with Sailkote and the "old" car glides beautifully. The next step is going to be filling whatever little nicks I have in the track with epoxy steel and file and sand everything smooth.

Also, not sure I'd use soap as a lube. Soap, by it's very nature, attaches to dirt.