I recently took my Mazda 3 to the Dealership for an oil change. They said something about changing the fluid in my trans.( 5 speed std) . Is this really needed....I've got over 50,000 miles on this 3.

 

It's only necessary if you feel sorry for them and want to give them some of your hard earned money. If they press you on the issue ask them to show you in the Mazda service manual where it says to replace the tranny fluid (Hint: it doesn't).

 

You might consider doing it yourself, if you are able, and converting to synthetic 75w-90?
You would need 3qts, an oil drain pan, and a 21mm (IIRC?) wrench or socket, plus a piece of hose to fit into the trans filler hole.

 

IIRC the manual recommends changing the trans oil at 60,000. If you feel any significant "notchiness" or live in an area that gets cold then you may want to consider swapping to synthetic.

 

I don't know what manual you're looking at, but I'm looking at the manual for my 2009 right now and it makes no recommendation for changing the gear oil for a manual transmission in a Mazda3 in either Schedule 1 or Schedule 2, regardless of time or mileage. Maybe older Mazda3s are different. Yes, no?

 

I'm with shipo. It's not in my manual either.

 

There is no recommendation in mine either (2007) though I did find where it says to inspect it, think it was like every 20k miles or something.
Personally i'd recommend changing it if you have 50k on the car, there has to be some grime from gears that's kicking around in there which can't help the life of the trans. I'm at ~34k and thinking of changing it just to be on the safe side... and see if it changes the mileage at all.

 

Grime from the gears? Highly unlikely. Why? Because manual transmissions don't "dirty" and/or "cook" their lubricant like engines and automatic transmissions do (no combustion gasses, no wet clutches and precious little heat generation). I've drained manual gearboxes after nearly 200,000 miles and the gear lube looked like the day it was installed at the factory.

 

new gears could easily have some unpolished edges that would chip off over time, all depends on the quality of the internals. Though remember that oil does break down with time & heat - how much i'm not sure, but i'd error on the side of safety. This is a price of 10 oil changes in the life of your ride, or in an extreme case over time a new transmission instead, take your pick.
I know a differential is different than a trans, but the rear end on my RX7 needed new lube yearly due to a previous owner not changing it at all. SO something was wearing / breaking down and dirtying up the oil.

 

A few bits of metal falling to the bottom of your tranny case are completely inert and will not dirty the gear lube in any way-shape-or-form. That said, my bet is that for every singly grain of metal from an "unpolished edge", there are dozens (if not hundreds) more from normal wear.

FWIW, "oil" will NOT break down at all with time and heat (at least not the heat generated by a manual transmission, even if it's used to tow or for racing). My guess is that you're thinking of engine oil where the additive package breaks down over time and with high applications of heat; the oil itself is typically good to go for a long-long time.

As for differentials, if we're talking about an open diff, then the gear lube should be good to go for the life of the vehicle, however, if we're talking about a clutch type limited slip differential, then yes, the oil needs to be changed on a periodic basis as the clutch material can in fact dirty the oil.