So, after all my posts, and going to get Toyota IV ATF, I found my manual for my 2001 S40. It says that the car was built in 10/00, has a AW 55-50 Transmission. and uses ATF Dexron III and Mercon Oil I am assuming (Duhh), not to use the T-IV, and go out and get some Dexron III !!! HELP! Any recommendations? It is in the middle of a Drain and Fill...car is now down 3qts of fluid. I don't think these fluids are compatable.

 

I'm sure you've figured this out by now but your tranny takes Toyota T-IV (JWS3309) fluid despite what the manual says.

 

Just want to make sure that the 'old' fluid, which remains in the tranny will mix with the new 3 quarts of T-IV that I will put in.

 

If the old fluid is JWS 3309 (T-IV), no problem. If it's Dexron-III, maybe, maybe not. The AW55-50 can operate with JWS 3309 or Dexron-III but Volvo says the two fluids should not be mixed.

Your car's manual suggests it might have come from the factory with Dexron-III. But the problem is more likely that the manual is incorrect and that your car actually had JWS 3309 installed at the factory.

I wouldn't sweat this nevertheless. JWS 3309 is just an improved Dexron-III. If your tranny shifts fine, leave it alone. If the tranny acts strange or you don't want to let it slide, do a complete fluid exchange for T-IV. It's easy to do - did it on my 2000 S40 recently. I used Walmart ATF, but T-IV can also be installed in the four speed AW 50-42 in my car, fwiw.

EDIT: On reflection, if this were my car I'd buy a case of 12 quarts of T-IV from the Toyota dealer and do a complete fluid exchange. T-IV should be about 5 bucks a quart from the dealer, btw, though some dealers will try and charge more. Call around.