As I do maintenance on my '00 RK I like to experiment to see what works or doesn't work, and maybe other folks can benefit. Although I definitely follow the mfr recommendations as a starting point, I do admit I'm more willing than most to experiment a little, and with all the wildly varying opinions of the internet I like to see for myself. I'm not trying to outsmart the engineers, but I do recognize that the mfr's interests are not always aligned with the user's interests (to put it kindly). Anyhow, I was due for a primary oil change, I had Valvoline Premium Blue 15w40 in there and it felt fine for the most part but after researching ATF in the primary I decided to give it a whirl. Lots of folks say ATF was like a night and day improvement in shifting and finding neutral, I was skeptical about that mainly because my bike already does that great, not sure how much better it could get but what the heck, let's see. I installed Valvoline Dex/Merc ATF, and although I don't have a lot of miles on it yet it feels about the same as before with shifting and finding neutral, which is to say pretty darn good. I was happy before, I'm happy now. Where I did notice an improvement was in the clutch action, with both Rotella and Valv Prem Blue I'd feel some minor coarseness right in the engagement zone, and with the ATF it was smooth all the way through. I don't know if it's the additives or the lower viscosity, but it's definitely smooth as silk (not that it was a problem before). I didn't notice any increase in primary chain noise, or any other problems. I have no way of knowing if it runs hotter or not. I'm going to run it 4-5k and see what the fluid and magnetic drain plug looks like. With the HDEO in there my drain plug had very little debris and the oil was pretty clean looking, so that's a decent baseline I think. At this point I think I could go either way with the ATF vs something like an HDEO oil from a feel perspective although the ATF for sure is a bit better. Anyway, maybe this helps some of you experimenters. One thing I'll say is if your bike is banging into gear or you can't ever find neutral you have a clutch adjustment problem more than a primary oil problem. But I do love me some Redline SPH in the tranny.