I have some 7-8 year old Dexron III (Mercon), both from Castrol and Proline, that I would like to use along with freshly bought Dexron VI. Does transmission fluid degrade over time or is it not a good idea to mix manuf or grades of Dexron in my 90 Camry?
The DEXRON III does not degrade in the bottle. The DEXRON VI is compatible with the DEXRON III. You can add the DEXRON VI to a system with DEXRON III but do not add DEXRON III to a system specifying DEXRON VI.
If you are doing a transmission fluid change, you can use the old DEXRON III and add some DEXRON VI, it might even provide a benefit, but if it does provide a benefit, then it will provide even more of this benefit if you use all DEXRON VI.