My GT350 comes with a Tremec TR3160 and it's spec'd by Ford for Mercon LV. My problem is notchy shifting from 1 to 2. 2 to 3 can be notchy too, but it's not as bad. The notchy sensation comes when you try to shift straight through from 1 to 2. If I pull against the gate for 2 without pausing, it's hard to get the gear engaged and sometimes it will grind. And this isn't speed shifting - my datalogger says I'm spending about 2 seconds on each shift. However, if I pull against the gate, relax for a moment and pull again, it engages easily on the second try. So far, Mercon LV is notchy and so is Dex 6, which is Tremec's spec for this gearbox. Now, before someone raises the "mechanical problem" point, it turns out that if I change the fluid, for about the first hour of use, all the way from cold to warm, it's brilliant - from gear to gear smoothly. Shifting with brand new ATF is perfect. It just doesn't stay that way. After a couple of days of driving, it's back to the old habits. So, I'm thinking of trying a GL4 and I'm wondering which one is most likely to work best: - Ford Motorcraft DCT fluid - XT-11-QDC which is about the same viscosity as Dex 6/Mercon LV - Redline PSF - a hair more viscous at 7 cSt. - RP Synchromax - I've tried it before and I haven't been impressed, but I'll try it again if it wins the bake-off - a DCT fluid (thinking about BMW MTF-LT-5): Pentosin FFL3/Ravenol DCT/Redline DCT etc. The key to success is going to be the right friction performance. ATF works for a short time, so I need something with the friction performance similar to brand new ATF that won't fade after an hour of use.