Mix synthetic with regular ATF?

[RiverRat37]

I'm getting ready to change my transmission fluid. I'm not sure if it has conventional or synthetic in it right now. From looking at the dipstick its not really dirty but I don't know when it was changed last. I've not owned it very long and I feel it should be changed. It shifts very smoothly and runs like new. My question is since I don't know what is in it now would it hurt to do a drain and fill with synthetic with me not knowing what is in it now? Meaning if it has regular ATF in it would it hurt to do a drain and fill and mix synthetic with the regular ATF? Also I'm going to be buying from a local Napa, Auto Zone, or Advance Auto... What is the best ATF for our 3rd Gens those stores carry? Thanks guys

 

[DC runner]

Not sure if ATF tech has followed Oil tech, but 15 years ago it was generally considered a no-no to mix conventional with synthetic oil.


Today it is less an issue. Synthetic oil has progressed to the point that is plays nicer with regular oil. As for how this compromises the synthetic oil's superior protection, not sure, but it won't decrease it; if that makes sense.

 

[maxspeed96ct]

I recommend doing the full flush , there's a write up on here somewhere. You need about 15-16 bottles. Its pretty easy a simple drain fill only cleans out a portion of you fluid.

I switched to full synthetic at 120k, and the truck just didn't seem to like it. Shifts were late and sloppy. After just 1 week I flushed again and went back to regular ATF fluid.

I know everyone raves about synthetic fluids but my old trans at 120k just didn't like it.

 

[binhsterbinh]

Go ahead and drain and fill with a synthetic of your choice. Fred Meyer I noticed has the best prices in my area for Valvoline Maxlife Dex/Merc ATF around $5.00-5.19 per quart while Mobil 1 Fully Synthetic Multi-Vehicle ATF is $6.50(usually $9.50 everywhere else). Maxlife is Synthetic despite not being labelled/marketed that way. Check the Valvoline site.

I ended up doing the full replacement using the ATF cooler line procedure with 16 quarts of ATF found in the tutorial section here. I did not bother dropping the pan or any of that. If you decide to drain just the pan, you should have no problem mixing synthetic to regular fluids. That'll take about 4.5 quarts I think. I did this method in my TL three times over a few weeks until old stuff was mostly out.

Redline D40 would be the best synthetic ATF and would pretty much last life of car while Maxlife or Mobil 1 ATF would suit just fine doing a 4-5 quart drain fill every few years.