To change
 

so what maintenance do you recommend at my mileage?
seafoam yes or no?

Yes change. Do the rear diff too while you're at it. Super easy. Search for some DIY's on here.

well first you never clarified if its an automatic or manual.
second mazda recommends you change diff and tranny fluid every 30k miles so yeah it wouldn't hurt
third like danny said do a search here on the forums for diys and fluid recommendations

Sweet jesus, if it's a manual, yes, change trans and diff fluid. Use GL4 in the trans, NOT GL5.
Gears don't slip, they grind when the lubricant is no longer doing its job -- like when it's 14 years old.

Seafoam is not more effective than water, but its also cheap enough that you may as well.

Things to consider changing unless they're recent:
- coolant
- brake fluid
- belts (at least inspect them)
- radiator
- examine underhood plastics (like the coolant reservoir) for cracking and degradation
- examine brake lines
- examine shocks
- air filter, cabin air filter
- examine oil injector lines and vacuum test the injectors themselves. This is a bit more complicated than the rest, but it's also a critical component to keep happy.
- examine catalytic converter for internal damage

all that you mentioned are perfectly fine.
and at 123,000 my car runs and shifts like butter right now so i want to keep it that way. No grinding or slipping what so ever. But i will look into doing a tran and diff fluid change

AAAANNNNDDD don't trust that your mechanic (for me the Mazda dealer) will put the correct MT fluid (GL4 rated fluid only) in your car. I went over to the dealer to get a jug filled of their bulk MT fluid ("the same kind we use on all MTs" they said - even gave them the mazda part #) - it turned out to be GL5. I replaced mine with Redline MT90 (GL4) at 42K miles and will likely do so again in 3yr/70K miles.

I dont trust any mechanic, i do all the work myself. Those damn mazda techs. Hows gl4 run during winter times?

I really didn't notice a change from the factory fill to MT90 last winter in New England nor the previous Winter in OH. That said - she's never seen a deep freeze Winter yet (Farmers Almanac and NOAA say this Winter is going to be much rougher temperature wise but not snow fall amounts compared to 2017).

[side note to my friends in Minnesota - look out your temperatures in the upper plains states are going to be brrrrrr cold]

It's fine in canadian winter. Obviously a bit thick while cold, but once it warms up there is no difference from summer. Pretty sure it's the same for most reasonable choices of oil.

Reminds me it is time to winterize the car. Will be doing these fluids along with coolant (using Mazda’s coolant) etc.

Yes. Change the trans and diff fluids. Redline MT-90 (GL4) in the trans and Redline 75W90 (GL5) in the diff worked great for me, although some people in colder climates report MT-90 as being a little notchy in very cold weather for the first few minutes. There is a Motorcraft branded oil that people like in the trans as well.

Despite what BMW may say, there is NO SUCH THING as a lubricant that lasts more than 100K miles. Not even Amsoil makes that claim.