No need to put the old oil back, but it could tell you something. It's probably too polluted to say anything about box condition but it's consistency will have established a reference that you are likely comparing too, as in, it sound different than before. Assuming that no mechanical shortfalls occured during oil change (contaniments)it may be that the new oil has flushed deposits within, in solvent fashion, and in fact may lead to excellerated wear of the box. Don't despair however as the box has likely given you good service for many years and just needs a little TLC to make sure it gives many more years of service. I would apply a couple of observations;
- audio, with a stethiscope to try and identify area of concern
- drain and refill oil after a week to inspect it's metallic content. Make sure you have high EP content in replacement oil.
- visual inspection if you have port access. Earlier the better so you have that reference.
- compitence, don't injure/pollute the box unintentionally.
The beauty of older boxes is the opportunity for repair. Gearsets can be expensive but bearing housings and shafting can typically be reworked to restore boxes without replacement costs.
