Yes, there is a spring in the transmission that returns the selector rod to center position. However, if the returning feature is not returning reliably, it could instead be something elsewhere in the linkage has too much friction. Generally speaking, to troubleshoot this I'd start with the external linkage (and you already mentioned lubricating the shift console in the passenger compartment, but did you also check that the cables were moving freely, and did you look at the bellcrank "contraption" back at the transmission, where it translates one cables motion to selector rod rotation? All those parts are external to the transmission.)
Edit: this thread has some pictures and info about the "contraption" (I'd recommend checking and lubricating yours if you haven't already - it does come into play when moving the knob left or right!):
http://986forum.com/forums/general-discussions/60643-excessive-shifter-play.html
After ruling out all the external sources, it's time to go internal, but this will probably get expensive and likely mean swapping or rebuilding the transmission. It's possible that some springs may be accessible under cover plates, although if something with the selector shaft and shift fork (inside the transmission) is worn out or bent, it would require transmission disassembly to completely diagnose it and repair. Best of luck!
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