ATF expands and what not. So the amount you drain may not be the same as the amount you fill. There has seemingly been instances of others doing this, having issues, going to the shop, and learning it was underfilled. But I am no expert at this.

I'm an advocate doing the confusing fill procedure instead of adding what you drain.

 

Pros
A lot cheaper than taking it to the dealer

Cons
Some fluid won't drain out (i.e. in the torque converter). To get this you will have to drain and fill several times and shift between the gears several times between each drain and fill.
Replacing exactly what you put in leaves you with the previous fluid level, which might not have been correct. However, if you haven't had prior issues with the fluid level, it stands to reason that you shouldn't have issues after.

Make sure you remove the fill plug before opening the drain plug. Nothing is worse than emptying out all the ATF only the realize that the fill plug is seized. Tapping the drain and fill plugs with a hammer can make freeing with a wrench easier.

 

I have done that process on my Camry, Rav4, and Highlander several times. Just make sure the fluid is cold and you accurately measure. I use a large 2 quart kitchen measuring cup that I got from Family Dollar. I measure the old fluid, wipe it clean, the measure the new fluid that I put in.
I use a funnel attached to clear tubing that I run into the full hole through the wheel well, and just gravity fill. Works great for me, I don’t like using pumps if I can avoid it.

 

You need to verify ATF level. To blindly assume that you have the correct amount of ATF right now is not too smart. I find too many Toyotas low on ATF. It is easy enough to check ATF with jumper wire or OBD2 scan tool and get the best correct ATF level and not some silly guestimate.

 

Honestly I think almost everyone who adds what they drain has low fluid. Which is why I rather champion doing it per procedure.

 

You guys are basing that on what? I guess I’m not too smart then, right Greasymechtech? I guess I got my engineering degree from a cereal box. If you measure accurately then it can’t be wrong.

 

I don't think he was denying your education but I have met so many doctorates and PhDs that can't even turn a wrench...fully licensed mechanics now can go to school and learn all the theory with compete in class hours then they come to an actual job for the 1st time and wonder why the engine took 80 liters of oil but the transmission dipstick still shows low ... 🤦‍♂️🤷‍♂️