I just acquired a 1978 Datsun 280Z. As per my usual, I have been very busy changing out all fluids for new. When I drained the rear diff, the oil was very dark in color, but with a very definite red tint to it. I have only ever used your normal brands of gear oil(Mobil , Valvoline, Shell, etc) and have never seen an oil for a diff that had a color like this. It was thick like a diff oil should be, so I doubt it was ATF. The fact that it is still running should prove that. I replaced it with the appropriate weight Valvoline synthetic gear oil, and it's quiet and smooth. Just curious as to what brand this may have been, as the previous owner was a little suspect in his maintenance routine. Just ask me about the possibly 31 yr old brake fluid I flushed outta this thing !