Transmission oil flush/change at 40,000 km?

buyingconstant7 said:

While I'm certainly not disagreeing with you, I'm more questioning it, but are you sure? I mean that's what many people said to do with synthetic oil, is if an engine has high miles and has been using a conventional oil, you should NOT switch to synthetic. But recent discoveries show that it doesn't matter if you switch between conventional or synthetic oil every oil change on a high mile engine. The same doesn't apply to transmissions?

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Well, I've messed up 3 transmissions changing their fluids later in life. One totally failed (Infiniti whatever is in a G20/Sentra SPEC V), one became jerky (Chrysler 545RFE), and one shifted poorer (Tremec T56). All changes by ASE Master or other dealer tech.

ETA: 4. Although the 4th could have been dead already, as the car sat up for a couple of years before I got it, but it exhibited IDENTICAL results to the G20/Spec V.--- Slip...engage...few weeks later no-shifting out of 1st.


Anyway, my cousin, a mechanic of many decades, told me this, and like a stubborn young man, I ignored him through 4 vehicles...the results are above.

Unobtanium said:

Well, I've messed up 3 transmissions changing their fluids later in life. One totally failed (Infiniti whatever is in a G20/Sentra SPEC V), one became jerky (Chrysler 545RFE), and one shifted poorer (Tremec T56). All changes by ASE Master or other dealer tech.

ETA: 4. Although the 4th could have been dead already, as the car sat up for a couple of years before I got it, but it exhibited IDENTICAL results to the G20/Spec V.--- Slip...engage...few weeks later no-shifting out of 1st.


Anyway, my cousin, a mechanic of many decades, told me this, and like a stubborn young man, I ignored him through 4 vehicles...the results are above.

Click to expand...

To the jerky/poor shifting, I've read where ASE mechanics forgot to add the friction modifier to non-OEM fluid cause those issues.

Triton46 said:

To the jerky/poor shifting, I've read where ASE mechanics forgot to add the friction modifier to non-OEM fluid cause those issues.

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Good thing I never used anything but OEM fluids from the dealers, and had the work done by the dealer, then.

buyingconstant7 said:

While I'm certainly not disagreeing with you, I'm more questioning it, but are you sure? I mean that's what many people said to do with synthetic oil, is if an engine has high miles and has been using a conventional oil, you should NOT switch to synthetic. But recent discoveries show that it doesn't matter if you switch between conventional or synthetic oil every oil change on a high mile engine. The same doesn't apply to transmissions?

Click to expand...

Essentially a transmission's fluid has additives/lubricants in it. After say 60k miles the fluid loses its formulated additive/lubricant properties. Lets view that 60k mark as the "fork in the road" with two paths:

A: At 60k miles and beyond lets say we don't change the fluid. As the fluid gradually loses its additive/lubricant properties more metal shearing takes place leading to metal shavings. In GM and ZF transmissions found in BMWs for example, said metal shavings (microscopic level) step in and now act as sort of the "new" lubricant additive. (Think about the metal additive Moly they put into engine oils). If one goes down this route say past 100k miles its too late to go back and change your fluids. If you do change the fluids you're draining out the valuable metal shavings the transmission has been relying upon to operate. You simply keep going down this route without changing the fluid. Your transmission may last 120k miles or may may last 200k miles. It just varies. Chances are you may sell the car before even experiencing failure.

B: A transmission fluid change at 60k miles means you eliminate significant metal wear from even occurring. You should also experience smoother shifting because the new fluid is fresh with additives/lubricants.

I currently own a Mazda5 with a 5-spd manual. I changed the transmission oil at 220,000km because I thought that I should, even though Mazda says lifetime oil. I noticed absolutely no change in how the transmission shifted. I now have 389,000km on the vehicle and it is still my daily driver. I'm discouraged to hear that so many are concerned about the "lifetime" fluid in the automatic transmission. I am considering purchasing a Mazda CX-5 next summer. I dearly want a stick-shift tranny, but the AWD version only comes with an auto. I'm sure Mazda isn't the only manufacturer of lifetime transmission fluids (Toyota Rav4, Honda CRV?,...) and I am wondering then, do any automatic transmissions last as long as manuals? Am I crazy to believe that I will go 389,000km in a new CX-5 with an auto? Will it die before then for sure?

Look at the "high mileage CX-5s check in thread"