differential service and Power steering belt and compressor belt installation

What is your question?

I have a 1998 Infiniti I30 w/204k miles on it. I've done a drop and drain on the tranny 3 times during it's life but the differential oil/fluid has never been changed. Should I do that now? does it use Tran fluid or gear oil? Is it ok to put synthetic version in it now? Does the differential fluid mix with the transmission fluid? also my power steering belt and compressor belts have only been replaced one time. I asked the most recent shop I took it to but they didn't do it and I forgot to ask again. I don't even see them. I thought there was only the serpentine belt. I assume one or both of these smaller belts are in a case. Should I replace them given the car is 23 years old and they've only been replaced one time?

What makes this problem better or worse?

No problems currently

How long has your 1998 INFINITI I30 had this problem?

It's just about service/maintenance/preventative action