I always check transmission oil level when the engine is warm and running (in park)
Follow the checking procedure in your user manual.

The text in this document is contributed by SeattleGS400 on CL. Thanks.

Tools required: 14mm socket and wrench, long 1.5 foot funnel with a small ½” tip (can get these black funnels from Schucks for $2), oil drain catch pan and newspaper sheets to catch errant oil spills, 2 jack stands, new crush washer (P/N 35178-30010 or P/N 35178-50010; approx. $2), 2 quarts Toyota T-IV ATF ($4-7 per quart, can buy from a Toyota dealership same stuff as Lexus)

NOTE: MAKE SURE THAT THE EXHAUST PIPES ARE NOT TOO HOT OR YOU WILL GET BURNED!!!

How to do it:

1) Chock (secure) the back wheels. Jack up the front of the car at the front jack points just behind the front wheels so you can get underneath it—do it for both sides.

2) Underneath the car, from the back of the front wheels two feet rearward, you’ll find the exhaust pipes join together into a “Y”. You’ll find a flat black pan just forward of the “Y” with a 14mm drain bolt head sticking out of it—that’s the ATF drain bolt. Place some newspaper and the oil drain catch pan underneath it, and remove the drain bolt to drain the ATF.

3) After about 5 minutes of draining, replace and securely install the drain bolt using a new crush washer. The ATF will still be coming out in a small drip stream—it’s OK, just replace the drain bolt.

4) In the engine bay, remove the ATF fluid level dipstick by twist unlocking it and pulling it straight out (it’s on the driver’s side of the engine). Place the tip of the funnel inside the exposed ATF fill hole, and slowly fill with 2 quarts Toyota T-IV ATF. Dump in the ATF too fast and it will leak out at the ATF fill hole--remember, it's only a 1/2" opening, so go slowly.

5) Replace the ATF dipstick and make sure that it’s in the lock position. Lower your car and remove the jack stands.

6) Start up the car. While pressing down the brakes fully, shift the car into R (leave there for 4 s), then to D (leave for 4 s), then to D4 (leave for 4 s), to D3 (leave for 4 s), to D2 (leave for 4s), then to L (leave for 4 s). Put the car in P, and leave it running.

7) Check the ATF level and add more fluid as necessary—don’t overfill. To properly check the ATF level, leave the car on, and while it's running, pull out the ATF dipstick, wipe it clean and reinsert it FULLY, then pull it out to check the level. As the car is COLD, look at the bottom "COLD" markers--the ATF fluid level should be around the top of the COLD markings. It might be a little bit above the COLD markings, as the car has been on for a few minutes (fluid expands!). However, if the car has been driven around for 10-15 minutes, you should use the HOT markings. In any event, you shouldn't need to add any fluid at all as the drain takes out exactly 2 quarts (what you just replaced!).

8) Drive around for at least 5 minutes (making sure that your car shifts into all gears for a least a little bit) to mix up the old and new fluid. Check for leaks underneath the car. If you'd like, you can now check the ATF level (as described above with the engine on), but use the HOT markings instead, as it's warmed up enough.

9) If doing another drain and refill process, repeat steps 1-8 again. You don’t need a new crush washer for each drain and refill, if you are doing a couple of them sequentially on the same day—just use a new one for the last drain and refill to ensure a leak-free seal.

HOW OFTEN SHOULD YOU DRAIN AND REFILL?

For each drain and fill process (drain the ATF, refill with 2 quarts, drive around town for at least 5 min to mix the new and old ATF), here’s how much of the old ATF fluid remains (the GS400’s A/T capacity is 9 quarts):

Percentage Old AT Fluid Remaining after a ATF Drain and Refill Process, and susequent Drain and Refills
(1) 77.8%
(2) 60.5%
(3) 47.1%
(4) 36.6%
(5) 28.5%
(6) 22.1%
(7) 17.2%
(8) 13.4%
(9) 10.4%
(10) 8.1%

Basically, after 10 drain and refill processes (20 quarts of ATF total), you will still have approximately 8% of the old fluid left, which is pretty neglible.

If you want essentially all of the old fluid out of the A/T in one step, then you’ll need to do a A/T flush procedure (moderate cost at Lexus dealerships or a rather challenging process for a DIY job as you don't have the macinery), but it does get all of the old fluid out. This is NOT recommended on cars with spotty service records, as sludge build-up will come loose as the new ATF's chemicals will get the sludge loosened, which can clog up some vital internal parts.

If your ATF is a brown or black color, I'd recommend maybe 4 sequential drain and refills at any one time--no more. Then do a single drain and refill of ATF along with each oil change interval (i.e. for the next 3 oil changes, drain and refill the ATF at that interval as well). This will allow your transmission to slowly clear up the sludge in a gentle manner and minize any clogging risk from large amount of sludge being loosened from "shock" of new ATF fluid.

For those cars that drain out their ATF and it comes out a raspberry red color, the ATF is in pretty good shape, and I would probably just do that 1 drain and refill. Then maybe once a year from there on out.

FYI, I do a drain and refill about 2x per year, just to ensure that I have some fresh fluid in there, while getting some of the old fluid out. I change my own engine oil (with synthtetic), so removing another drain plug for the ATF fluid is no hassle for me (and realizing the savings can be significant!). It's probably overkill replacing my ATF 2 times per year, but I look at it as some relatively cheap transmission maintenance ($15 each drain and refill), as I'll be keeping my car for a few years. . .

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