So I bought my FD with 120,000 ish miles... with 40k on motor. I wanted to just play it safe & replace the fluids, since I have no idea how old the stuff in the car is. So... what fluids should I replace... & what are not really necessary?

I'm thinking oil (which I already obviously do)... coolant, & tranny. Anything else I should do? I'd rather spend a little bit of $ on new fluids, & not have to deal with problems from bad fluids later on.

 

The diff and the power stearing. I like red line so I use that in my diff and trans. also use there water wetter in the coolant.

 

IMO, they're all "necessary", generally easy and relatively inexpensive. I'd vote for the brake fluid too.

 

Your instincts are correct, replace all fluids right away. Some of my personal recommendations would be Mazda long life anti-freeze from the dealer and synthetic fluids
in the differential and transmission. Make sure the oil filter has an anti-drainback valve....I use the stock Mazda filter, and get a new crush washer for the oil drain plug.

Also at the top of the list should be changing the fuel filter. A clogged filter can disrupt the fuel flow and possibly detonate your engine. If you can't be sure it was changed relatively recently I'd change it anyway as a precaution.