Many of us have been running Mercon SP ATF in our ZF 6HP26 transmissions for years with no issues. My wife's 2006 XK8 certainly fits that category....

Just learned of another ATF that I would probably switch to if Mercon SP is eventually discontinued here in the U.S. It is called Top Tech ATF 1800 Liqui Moly LM20032, currently selling at fcpeuro.com for $6.99 per litre. It was supposedly created in Europe especially for the ZF 6-speed and 8-speed transmissions and as expected, it meets the required Shell M1375.4 specs....

Those of you planning an ATF drain-and-fill may want to check out this product before you make your final ATF choice....

 

My main concern would be the effects of mixing the two products. I did some research and the instructions for use of TOP TEC ATF 1800 states: "The specifications and instructions from the assembly or vehicle manufacturer must be followed! Optimum effectiveness is only possible when the product is used unmixed." This means that a complete flush would have to be done, and it's not always advisable to to that with older cars.

I was only able to find one review from a BMW owner and he said it worked fine IMMEDIATELY after changing his transmission fluid. Think I will wait for a few more reviews on it first before trying.

 

I view that verbiage as just a statement for any future legal protection by the ATF manufacturer. As long as the fluid indeed meets the required Shell M1375.4 specs, my concern regarding mixing fresh fluid with old fluid would be minimal. The two drain-and-fills of Mercon SP in my wife's 2006 XK8 have certainly long since mixed with whatever remains of the original Lifeguard 6 with no apparent issues. Plus, these vehicles are now worth just a slight fraction of their original value and are light years beyond any type of warranty, therefore the risk is negligible as far as I am concerned....

That said, we know Mercon SP works well and I plan to stick with it as long as it is available and reasonably priced....

 

+1 Miragepro and +1 Jon89

As a ZF5HP24 owner, I couldn't get any of the recommended ATF fluids and certainly wouldn't pay for the Lifeguard5 to be custom flow in just for me.
So I had to bite the bullet for Liqui Moly ATF1200 (note for ZF5 gearbox). No problems with it so far.

Miragepro - certainly agree that a complete ATF flush is not advisable on our old sensitive transmission units. Agree with Jon89 that those words on the instruction is just legal worded "read the small print" garb#$e!

Jon89 - agree that there'll always be an element of mixing old with new due to the torque converter (& new fluid will always have different properties than the old burnt fluid anyway). FWIW You always have to get in there later (doing ATF refill twice) anyway to make sure you top up/drain to make sure you have the right level. I initially put in 6L first time and ended up with 7L in second time round (no idea why the difference - not an exact science with hot ATF dripping down to your armpits).

So it's always good to know that you have options.

 

I'm currently running that ATF in my transmission, and have been for the past 1,200 miles since February. When I posted about it in the forum, it prompted the outcry from nearly everyone in the thread. I decided to give the Liqui-Moly Toptec 1800 a try for a few reasons, chiefly price, lack of availability of Mercon SP in my area, and the BMW guys who have the same transmission speak highly of this fluid.

So far my experiences have been very positive. My shifts are much smoother than they used to be and they also happen with little to no delay. The delay when shifting into reverse after a spirited drive has also been reduced significantly. The 2-1 bump I would get has been reduced significantly to the point of being nearly imperceptible.

Before the drain and fill (I did also change the filter, mechatronic connector sleeve, and the valve body seals), every other week, the car would sometimes slam on a 3-4 or 4-5 upshift and prompt a "Gearbox failure" warning message with the amber warning light. Turning off and then on the ignition would clear the message. Three weeks after the fluid change, this happened again, but the slam was more of a push, it was nowhere nearly as violent and the messages returned. Again, cycling the ignition would resolve it. Before switching to Liqui-Moly, the delay into reverse could sometimes take up to a full second.

During the next 2 months, this behavior was becoming less and less frequent, and each time the slam was less and less violent. The last few times it happened, I would get the Gearbox warning with no perceptible change in how the car shifted. Not a slam, push, shove, or even hiccup to be felt. Again, cycling the ignition solved it.

Now, nearly in September, and after 1200 miles the issue seems to have been resolved. I have not had the slam since April, and the warning message since June. The delay into reverse after a spirited drive is now gone. Thus far, I do recommend using the Liqui-Moly.

 

Thanks for your review. You answered my concern regarding mixing the two fluids.