trans fluid and diff oil

[sfam198]

I have a two part question. I have a '98 F-150 4x4 with manual transmission. It's got almost 40,000 miles on it, and the manual trans fluid has never been changed that I'm aware of. The normal schedule calls for a 60,000 mile change, severe schedule calls for 30,000 miles. I want to change it to a synthetic fluid. It calls for Mercon fluid. What would be the best (and most readily available) fluid to use? I don't want to order anything, I want to be able to go to the store and just buy it, so Amsoil, etc. is out of the question. Also, how long would this fluid be good for after I changed it with normal driving conditions? I will also be changing out the fluid in the transfer case with the same fluid/interval.

Next question. My rear diff is filled with Ford's "lifetime" oil 75w-140 synthetic. The service interval according to Ford is 100,000 miles for either severe or normal service. Is there any reason to change it out early, or should I just wait until 100,000? Also, the front diff is filled with 75w-90 conventional oil. A change interval is not listed in either schedule. Is it the same as the rear diff? Should I change it out early? Should I keep it conventional or use a synthetic. The 4x4 hardly ever gets used, only occasionally in the winter, sometimes when I'm out on the trail.

 

[unDummy]

You're not going to find too many 'mercon' only fluids or upgrades. 30k should be a goal for the transmission whether severe service or not. I am one that doesn't have faith in OEM recommendations. Boutique ATFs or MTLs/MTLs are your best choices even though local autoparts stores don't have them. I'd wager that plenty 4x4 or speed shops will have the premium fluids like Redline/Amsoil MTF/ATF/MTL....

Besides the mercon, pennzoil/texaco have synchromesh/mtx that are sometimes at the parts stores(autozone/advancedauto). Another ATF substitute is royalpurple synchromax(Pepboys/napa).

I would also change out the front/rear-end gear oil. 1/2 the OEM recommended interval(50k) is a good starting point. Interval also depends on driving style and time(9 years is too long on fluid). Stick with synthetics front and rear for optimum performance and longevity. Change fluid every 3-4 years if mileage isn't climbing quickly enough.

Don't forget brake bleeding, the PSF, and the x-case.

 

[sfam198]

thanks for the advice. after doing some more reading on the subject, it seems that amsoil may be my best choice for both transmission and axles. i can't find much in the way of synthetic trans fluid or geear oil around here for some reason. the only thing i really did see was b&m trick shift at AAP, but i don't really want to use that in my manual trans.

i will probably go with the amsoil universal atf for the trans and t-case (both will be changed at the same time) and amsoil 75w-140 for the rear axle and 75w-90 for the front.

i have to do a brake job pretty soon, so i will be flushing the brake fluid at that time, probably no more than a month or 2 from now. what kind of reccomendation is there for psf? according to the manual, it takes mercon atf, so should i use the same amsoil atf? ford power steering pumps are junk, so i would think the synthetic fluid would increase the life of the system a bit, but is it just a waste of money to use the good synthetic in there?

 

[Tempest]

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is it just a waste of money to use the good synthetic in there?

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I put Amsoil ATF in my Ranger P/S with 70+k on the clock. It likes the fluid a lot.